Calendar reforms didn’t just tidy up dates—they erased days from people’s lives. In 1752 Britain dropped 11 days; in 1918 Russia skipped 13. These changes rippled through public holidays, quarter days, rents, and the anniversaries we mark today. Understanding how Old Style (Julian) and New Style (Gregorian) dates map to each other will help you read On This Day entries, interpret historical sources, and align commemorations on a modern calendar.

What does it mean that “11 days vanished”?

When a country adopted the Gregorian calendar—created to fix the Julian calendar’s drift—its government decreed that a block of dates would be omitted to realign the calendar with the seasons. In Britain, Wednesday 2 September 1752 was followed by Thursday 14 September 1752, instantly removing 11 days. Russia did the same in 1918, jumping from 31 January to 14 February, dropping 13 days.

Why the world changed calendars

The Julian calendar, introduced by Julius Caesar, miscalculated the solar year by about 11 minutes. Over centuries that small error accumulated, shifting the calendar away from the solar year and moving Christian festivals tied to the equinox. Pope Gregory XIII introduced the Gregorian calendar in 1582, revising leap-year rules and deleting dates to reset the calendar to the spring equinox.

  • Gregorian leap-year rule: years divisible by 4 are leap years; century years must be divisible by 400 to be leap years (e.g., 1600 and 2000 yes; 1700, 1800, 1900 no).
  • Initial correction: countries adopting in 1582 skipped 10 days; those adopting later skipped more (11 in the 1700s, 12 in the 1800s, 13 in the 1900s).

How different countries “lost” days

Britain and its empire, 1752

The Calendar (New Style) Act 1750 made two major changes that took effect in 1752:

  • New Year’s Day moved from 25 March to 1 January for legal and civil purposes.
  • Eleven dates were skipped: 2 September 1752 was followed by 14 September 1752.

Contrary to colorful folklore, there’s little evidence of widespread “Give us our eleven days!” riots. But the change did disrupt how people paid rent, held hiring fairs, and observed saints’ days. It also created confusion around dates in early 18th-century records, which sometimes appear as double-year entries (e.g., 11 February 1731/32).

Russia, 1918

Following the Bolshevik decree of 24 January (O.S.) 1918, Russia moved from the Julian to the Gregorian calendar. Thursday 31 January 1918 (Julian) was followed by Friday 14 February 1918 (Gregorian), skipping 13 days. Civil life adopted the new calendar, while the Russian Orthodox Church continued to use the Julian calendar for fixed feasts, which is why many Orthodox communities celebrate Christmas on 7 January (Gregorian), the Julian 25 December.

This switch also affected commemorations: the “October Revolution” of 25 October 1917 (O.S.) is marked internationally on 7 November (N.S.).

Sweden’s false start—and 30 February

Sweden planned a gradual shift by omitting leap days over 40 years (1700–1740). The plan faltered; to reset, Sweden added an extra leap day in 1712, creating 30 February 1712. Sweden then adopted the Gregorian calendar outright in 1753 by skipping 11 days (17 February was followed by 1 March).

Early adopters in 1582

Spain, Portugal, Italy, and the Polish–Lithuanian Commonwealth adopted the Gregorian calendar immediately. They skipped 10 days: Thursday 4 October 1582 was followed by Friday 15 October 1582. Some German states and the Netherlands followed soon after, others later.

Other notable changeovers

  • Japan (1873): Switched from a lunisolar system; the day after 2 December 1872 became 1 January 1873.
  • Greece (1923): Thursday 16 February (Julian) became Friday 1 March 1923 (Gregorian).
  • Turkey (1926): Adopted the Gregorian calendar for civil use on 1 January 1926.

The ripple effects: holidays, fairs, rents, and anniversaries

Public holidays and saints’ days

When dates shifted, communities faced choices:

  • Keep the name-date (e.g., always celebrate on “5 November” regardless of astronomical drift).
  • Keep the seasonal timing by moving the event the number of days skipped (e.g., celebrate “Old May Day” 11–13 days later).

In Britain after 1752, some rural areas continued with “Old” festivals:

  • Old Christmas: After the 1752 shift, Julian 25 December mapped to 5 January (Gregorian). With the offset increasing to 12 days in 1800 and 13 in 1900, some traditions slid to 6 or 7 January.
  • Old Twelfth Night: Wassailing customs often moved to 17 January in some regions.
  • Old May Day: Observed around 12–13 May in communities that kept the older rhythm.
  • Old Michaelmas Day: Marked around 10–11 October instead of 29 September.

Movable feasts (like Easter) were recomputed under the Gregorian rules, so they realigned with the spring equinox rather than sliding through the seasons.

Quarter days, rents, and the British tax year

British financial life ran on “quarter days” when rents, wages, and contracts often fell due:

  • Lady Day (25 March)
  • Midsummer (24 June)
  • Michaelmas (29 September)
  • Christmas (25 December)

After 1752, to preserve financial intervals, many obligations slid by the number of omitted days. A famous legacy is the UK tax year:

  • After the 1752 reform, the start moved effectively to 5 April (11 days after 25 March).
  • Because 1800 was a leap year in the Julian calendar but not in the Gregorian, the offset grew by one, and the start shifted to 6 April, where it remains.

Leases and rents often referenced “Old Lady Day” and other old-style quarter days. Many local hiring “mop” fairs similarly moved 11 days later; some still carry “Old” in their names even if the date has been modernized.

Fairs and hiring days

Market towns anchored fairs to saints’ days or term days. After the reform, organizers had to choose between keeping the date label (e.g., “St. Bartholomew’s Fair” on 24 August) or the harvest season context. In practice, many fairs slid forward by 11 days; others eventually re-synced to civic calendars. Echoes of these choices remain in “Old” fairs still held in mid-October (near Old Michaelmas) or mid-May (near Old Whitsun/May Day).

Birthdays and national anniversaries

  • George Washington: Born 11 February 1731 (O.S.) in Virginia, which became 22 February 1732 (N.S.) after adjusting both the 11-day shift and the New Year change from 25 March to 1 January.
  • William Shakespeare: Traditionally 23 April 1564 (O.S.). Converting exactly yields 3 May 1564 (N.S.), but England celebrates 23 April, aligning the cultural observance to St. George’s Day rather than converting.
  • October Revolution: 25 October 1917 (O.S.) is 7 November 1917 (N.S.), the date now used internationally.
  • Guy Fawkes Night: Stayed on 5 November (by name), not shifted 11 days; a case of retaining the labeled date rather than the seasonal position.

How to read “Old Style” vs “New Style” dates (especially in On This Day entries)

Old Style (O.S.) usually means a date recorded in the Julian calendar and, in English records before 1752, possibly with the year starting on 25 March. New Style (N.S.) means the Gregorian calendar with the year beginning 1 January.

Quick rules of thumb

  • Identify the place and date: The offset depends on when a country adopted the Gregorian calendar.
  • Apply the correct day offset for Julian-to-Gregorian conversion at that time:
    • 1582–1699: add 10 days
    • 1700–1799: add 11 days
    • 1800–1899: add 12 days
    • 1900–2099: add 13 days
    • 2100–2199: add 14 days
  • Check the New Year boundary: In England and Wales before 1752, dates from 1 January to 24 March are often written as double years, e.g., 11 February 1731/32. The modern (N.S.) year is the latter.
  • Watch for ecclesiastical vs civil calendars: Orthodox churches may still use Julian dates for fixed feasts; civil records use Gregorian in countries that adopted it.

How “On This Day” entries handle O.S. and N.S.

  • Best practice: Show both when relevant, e.g., “7 November 1917 (25 October O.S.)—October Revolution begins.”
  • For pre-1752 English dates: If you see 11 February 1731/32, modernize to 11 February 1732 (N.S.), and optionally note the O.S. year.
  • For biographical entries: Indicate any conversion, e.g., “born 22 February 1732 (11 February 1731 O.S.).”
  • Avoid proleptic confusion: Some databases render all dates in the proleptic Gregorian calendar (as if it always existed). If you do that, label it clearly to prevent misinterpretation.

Worked examples

  • Britain, 1752 conversion: A diary date of 10 September 1752 (Julian) has no civil equivalent—those dates didn’t exist. The next day was 14 September 1752 (Gregorian).
  • Washington’s birth: 11 February 1731 (O.S., year starting 25 March) becomes 22 February 1732 (N.S., add 11 days and roll New Year to 1 January).
  • Russia, 1917–1918: 1 February 1918 (O.S.) maps to 14 February 1918 (N.S.), adding 13 days.

Aligning historical observances on modern calendars

Decide what you’re preserving

  • By intention: Preserve the seasonal or liturgical moment. Move the observance forward by the appropriate offset (11–13 days, depending on the era).
  • By label: Preserve the named date (e.g., 23 April for Shakespeare), accepting that it sits slightly differently in the solar year than it originally did.

Practical approach for historians, genealogists, and curators

  • State both forms on first mention: “Old May Day (1 May O.S.; 12/13 May N.S.).”
  • Use current civil dates for event listings but footnote the O.S. origin to help readers connect the dots.
  • For multi-century traditions, note offset changes: 11 days in the 1700s, 12 in the 1800s, 13 from 1900 to 2099. That explains why some “Old” festivals migrated again over time.
  • When in doubt, cite the jurisdiction and the calendar explicitly: “10 October 1752 (Old Michaelmas, calculated as 29 September O.S. + 11 days in Great Britain).”

Myths and misconceptions

  • “People rioted to get their days back.” Satirical art popularized this idea, but firm evidence for widespread riots is scant. There was grumbling, confusion, and isolated unrest, but the transition was largely orderly.
  • “Old Style” always means Julian vs Gregorian. In English records, O.S. can also refer to the old New Year beginning on 25 March. That’s why double dating appears in January–March entries.
  • “One conversion fits all.” Offsets vary by century and by country’s adoption date. Always anchor your conversion to time and place.

Quick reference: which days were skipped where?

  • Italy, Spain, Portugal, Poland-Lithuania (1582): 4 October → 15 October (skip 10 days)
  • British Isles and colonies (1752): 2 September → 14 September (skip 11 days); New Year moved to 1 January
  • Sweden (1753): 17 February → 1 March (skip 11 days)
  • Russia (1918): 31 January → 14 February (skip 13 days)
  • Greece (1923): 16 February → 1 March (skip 13 days)

SEO cheat sheet: searching and comparing dates

  • Keywords to use: calendar reforms, Julian vs Gregorian, Old Style vs New Style, date conversion, double dating, skipped days, Gregorian adoption.
  • Search tip: Add the country and year (e.g., “Britain 1752 calendar act,” “Russia 1918 Gregorian”).
  • For On This Day: search both “O.S.” and “N.S.” versions, especially for events in Russia (1917–1918) and Britain (pre-1752).

Bottom line

When 11 days vanished, people didn’t stop celebrating—they adapted. Some kept the labeled date; others kept the seasonal moment. If you identify the place, apply the right offset, and note the New Year shift where relevant, you can read Old Style vs New Style dates confidently and schedule historical observances in a way that honors both tradition and accuracy.

FAQ

Why did Britain skip 11 days in 1752?

To adopt the Gregorian calendar and realign dates with the seasons. The Julian calendar had drifted, so Parliament ordered that 2 September 1752 be followed by 14 September 1752, omitting 11 days.

What’s the difference between Old Style and New Style?

Old Style (O.S.) generally refers to the Julian calendar and, in English records, the old New Year on 25 March. New Style (N.S.) uses the Gregorian calendar and 1 January as New Year’s Day.

How do I convert an Old Style date to a modern date?

Determine the place and year, add the correct day offset (10, 11, 12, or 13 days depending on the century), and adjust the year if the date falls between 1 January and 24 March in pre-1752 English records. For example, 11 February 1731 (O.S.) becomes 22 February 1732 (N.S.).

Why is Russian Christmas on 7 January?

The Russian Orthodox Church follows the Julian calendar for fixed feasts. Julian 25 December currently corresponds to 7 January in the Gregorian calendar (a 13-day difference in the 20th and 21st centuries).

Why does the UK tax year start on 6 April?

After the 1752 calendar reform, the fiscal start moved to 5 April to preserve the interval from the old Lady Day (25 March). In 1800, an additional day was added to keep alignment, shifting the start to 6 April.

What is double dating (e.g., 1731/32)?

Before 1752 in England and Wales, the legal year began on 25 March. Dates from 1 January to 24 March were sometimes written with two years (e.g., 1731/32). The modern year is the latter: 1732.

How should I list dates in On This Day entries?

Use the modern (N.S.) date for clarity, and include the O.S. in parentheses if it’s historically significant. Example: “7 November 1917 (25 October O.S.)—Bolsheviks seize power in Petrograd.”