Weekday names carry a hidden story: they’re a blend of ancient astronomy, local gods, and religious reforms. Most come from two systems—planetary-god names and numbered days—layered onto a seven-day week that spread from the Near East to Rome and beyond. Understanding these roots helps explain why Thursday nods to Thor, why jeudi points to Jupiter, and why Portuguese counts its feiras.

Below is a clear, comparative tour of where the days of the week get their names, how languages diverged, and how those choices still impact calendars, translations, and “On This Day” timelines.

From sky lore to schedule: why seven days at all?

The seven-day week grew from a fusion of astronomy, religion, and administration:

  • Seven classical “planets”: In antiquity, “planet” meant the Sun, Moon, and five wandering stars visible to the eye: Mars, Mercury, Jupiter, Venus, and Saturn. Their prominence helped shape a seven-day cycle.
  • Jewish Sabbath tradition: A seven-day rhythm anchored by a day of rest spread via the Roman Empire after the first century CE, gradually displacing Rome’s older eight-day market cycle (the nundinae).
  • Astrological hours: Late antique astrology associated each day with a ruling planet (and deity), reinforcing a planetary naming scheme.

Rome’s planetary blueprint

Latin locked in a template that many languages still wear:

  • dies Solis (Sun’s day) → Sunday
  • dies Lunae (Moon’s day) → Monday
  • dies Martis (Mars’s day) → Tuesday
  • dies Mercurii (Mercury’s day) → Wednesday
  • dies Iovis (Jupiter’s day) → Thursday
  • dies Veneris (Venus’s day) → Friday
  • dies Saturni (Saturn’s day) → Saturday

In Christianized Latin, Sunday often shifted from dies Solis to dies Dominica (the Lord’s day), and in some areas Saturday moved toward sabbatum (Sabbath), changes still visible in Romance languages.

How Norse and Germanic gods stepped in

When the naming scheme reached Germanic-speaking peoples, they translated the Roman gods into their own—interpretatio germanica—yielding today’s English patterns. The mapping is striking:

  • Sunday: Sun’s day (untranslated celestial body)
  • Monday: Moon’s day
  • Tuesday: Tiw’s/Týr’s day (war god), matching dies Martis
  • Wednesday: Woden’s/Odin’s day, mirroring Mercury (dies Mercurii)
  • Thursday: Thor’s day, matching Jupiter (dies Iovis)
  • Friday: Frigg’s (or possibly Freyja’s) day, paralleling Venus (dies Veneris)
  • Saturday: Saturn’s day (the Roman god survives intact)

German takes a related route, with one notable reform: Mittwoch (Wednesday) replaced Woden’s day with “mid-week” under Christian influence. Dutch keeps planetary echoes (donderdag, Thursday) while also showing regional shifts.

Romance languages: mostly planets, with two religious edits

French, Spanish, and Italian preserve the planetary Latin core, but Sunday and Saturday show Christian overlays:

  • French: lundi (Moon), mardi (Mars), mercredi (Mercury), jeudi (Jupiter), vendredi (Venus), samedi (Sabbath), dimanche (the Lord’s day)
  • Spanish: lunes, martes, miércoles, jueves, viernes, sábado (Sabbath), domingo (Lord’s day)
  • Italian: lunedì, martedì, mercoledì, giovedì, venerdì, sabato (Sabbath), domenica (Lord’s day)

The standout exception in the Romance family is Portuguese, which intentionally abandoned pagan-planetary names for most weekdays.

Portuguese: the numbered feiras

In Portuguese, weekdays are numbered and paired with feira (“weekday, fair day”), a Christian liturgical term popularized in the early medieval period:

  • segunda-feira (Monday) – “second [weekday]”
  • terça-feira (Tuesday) – “third”
  • quarta-feira (Wednesday) – “fourth”
  • quinta-feira (Thursday) – “fifth”
  • sexta-feira (Friday) – “sixth”
  • sábado (Saturday) – Sabbath (retained)
  • domingo (Sunday) – Lord’s day (retained)

This system counts from Sunday as the first day of the week (hence Monday as “second”). It’s a prime example of religious reform reshaping the naming of days without changing the seven-day cycle.

Hebrew and Arabic: numbered days anchored by Sabbath and Friday prayer

Semitic languages often number the days, with special names for the weekly holy day:

  • Hebrew: Yom Rishon (1st, Sunday), Yom Sheni (2nd), Yom Shlishi (3rd), Yom Revi’i, Yom Chamishi, Yom Shishi (6th, Friday), Shabbat (Saturday)
  • Arabic (many varieties): al‑ahad (1st, Sunday), al‑ithnayn (2nd), ath‑thulāthā’ (3rd), al‑arbi‘ā’ (4th), al‑jumu‘a (Friday, “gathering” for prayer), as‑sabt (Saturday, Sabbath)

The Hebrew and Arabic systems highlight religious rhythm: Sabbath and Friday prayer structure the week and the vocabulary.

South and East Asia: planets thrive, numbers rise

Indian subcontinent: the graha days

Indic languages preserve a planetary set aligned with Hindu astrology:

  • Ravi‑vāra (Sun), Soma‑vāra (Moon), Maṅgala‑vāra (Mars), Budha‑vāra (Mercury), Guru/Bṛhaspati‑vāra (Jupiter), Śukra‑vāra (Venus), Śani‑vāra (Saturn)

These names parallel the Greco-Babylonian planetary order. Historical exchange across Central Asia and the Hellenistic world helped standardize this familiar seven.

Japan and Korea: planets via the five elements

Japanese and Korean weekday names retain a planetary logic filtered through the five-element cosmology:

  • Japanese: nichiyōbi (Sun), getsuyōbi (Moon), kayōbi (Fire/Mars), suiyōbi (Water/Mercury), mokuyōbi (Wood/Jupiter), kin’yōbi (Metal/Venus), doyōbi (Earth/Saturn)
  • Korean: il‑yoil (Sun), wol‑yoil (Moon), hwa‑yoil (Fire/Mars), su‑yoil (Water/Mercury), mok‑yoil (Wood/Jupiter), geum‑yoil (Metal/Venus), to‑yoil (Earth/Saturn)

Chinese and Vietnamese: the triumph of numbers

Modern Chinese broadly uses numbered weeks: 星期一 (Monday, “week day one”), up to 星期天/日 (Sunday). Vietnamese follows a similar pattern: thứ hai (Monday, “second”), through thứ bảy (Saturday), with Chủ nhật (Sunday) as a special case. The planetary set exists historically in East Asia, but numerals won out in everyday Chinese and Vietnamese.

Slavic and Greek: numbers and descriptors

Many Slavic names blend numbering with descriptors tied to the Sabbath:

  • Russian: ponedélnik (after “no-work” day), vtórnik (second), sredá (middle), chetvérg (fourth), pyátnitsa (fifth), subbóta (Sabbath), voskrésen’e (resurrection, Sunday)
  • Polish: poniedziałek, wtorek, środa, czwartek, piątek, sobota, niedziela (no-work/Sunday)

Greek blends numbering with Christian terms: Δευτέρα (Monday, second), Τρίτη (third), Τετάρτη (fourth), Πέμπτη (fifth), Παρασκευή (Friday, “preparation” for Sabbath), Σάββατο (Sabbath), Κυριακή (Lord’s day).

What the names reveal about calendars

Which day starts the week?

The naming system hints at the week’s start:

  • Sunday-first cultures: Hebrew (Yom Rishon), Arabic (al‑ahad), and Portuguese numbering (with Monday as “second”) treat Sunday as Day 1.
  • Monday-first standard: ISO 8601 (used widely in Europe and in many software systems) labels Monday as day 1 and Sunday as day 7, regardless of names.
  • Mixed practice: English-speaking calendars often display Sunday first in print and online, even when software stores weeks as Monday-first internally.

Planetary vs numbered systems: how they affect translation

In multilingual contexts, day names can mislead if you assume direct cognates. A few pitfalls and tips:

  • Portuguese “quarta‑feira” is Wednesday (fourth day), not the fourth day of a Monday‑first ISO week; it presumes a Sunday start historically.
  • French jeudi and English Thursday are planetary cousins: both trace to Jupiter’s day (dies Iovis), via Thor in English.
  • German Mittwoch breaks the planetary chain—“mid‑week”—so don’t expect Mercurial roots here.
  • Arabic al‑jumu‘a (Friday) isn’t numbered; it’s named for congregational prayer.

How this shows up in “On This Day” listings

Historical timelines and “On This Day” pages often localize the day name. That can create subtle issues:

  • Abbreviation drift: “Thu” in English maps to jeudi in French and quinta‑feira in Portuguese; automated systems must rely on locale codes, not string matching.
  • Week starts shift: A dataset keyed to ISO Monday‑first might be displayed in a Sunday‑first UI. Events can appear under a different column while still referencing the same date.
  • Religious days anchor narratives: Entries like “signed on a Friday” may be rendered as al‑jumu‘a in Arabic or sexta‑feira in Portuguese—different words, same weekday.
  • Cross‑platform syncing: iCalendar (ICS) and ISO standards use numeric day codes (e.g., MO, TU), which helps avoid the ambiguity of names during translation.

A quick cross‑language snapshot

  • English (Germanic‑Norse): Sun, Moon, Tiw/Týr, Woden, Thor, Frigg/Freyja, Saturn
  • French/Spanish/Italian (Romance): Moon, Mars, Mercury, Jupiter, Venus, Sabbath, Lord’s day
  • Portuguese: numbered feiras (Mon–Fri), plus Sabbath and Lord’s day
  • Hebrew: first through sixth, plus Sabbath
  • Arabic: first through fourth, Friday congregation, Sabbath
  • Indic languages: Sun/Moon and five planets (grahas)
  • Japanese/Korean: Sun/Moon plus five elements/planets
  • Chinese/Vietnamese: numbered days dominate today

Why some days feel familiar across languages

You’ll spot patterns even across unrelated languages because the planetary seven and the seven-day liturgical cycle traveled widely. Whether you say Thursday, jeudi, or quinta‑feira, you are pointing to the same ancient scaffolding—one that fused sky-watching, local gods, and weekly worship into the calendar we still live by.

Key takeaways

  • The seven-day week is ancient, shaped by visible “planets” and the Sabbath rhythm.
  • Two dominant naming styles: planetary/god-based (Latin → Germanic/Romance, Indic, Japanese/Korean) and numbered (Hebrew, Arabic, Portuguese, Chinese, Vietnamese, Greek).
  • Religious reform often renamed Sunday and Saturday (Lord’s day, Sabbath).
  • Localization matters: weekday names don’t translate one-to-one; software relies on codes and locale rules.

FAQ

Why are there seven days in a week?

Because ancient cultures combined a seven-body sky model (Sun, Moon, five visible planets) with religious cycles like the Sabbath. Rome adopted the seven-day rhythm in late antiquity, and it spread with Christianity and imperial administration.

Why is Thursday named after Thor?

English translated the Latin dies Iovis (Jupiter’s day) into the nearest Germanic equivalent: Thor, the thunder god. Likewise, Tuesday aligns Tiw/Týr with Mars, Wednesday links Woden/Odin with Mercury, and Friday associates Frigg (or Freyja) with Venus.

Why does Portuguese use numbers (segunda‑feira, terça‑feira)?

Portuguese replaced most planetary names with numbered feiras under Christian influence, counting from Sunday as the first day. Saturday (sábado) and Sunday (domingo) kept their Sabbath and Lord’s day roots.

Do all languages name days after planets or gods?

No. Many do (Latin, Germanic, Indic, Japanese/Korean), but others prefer numbers (Hebrew, Arabic, Chinese, Vietnamese, Greek) or descriptors tied to the Sabbath cycle (many Slavic languages).

Which day starts the week: Sunday or Monday?

It depends on context. ISO 8601 starts on Monday; many European schedules follow that. Religious and cultural traditions (Hebrew, Arabic, Portuguese) often treat Sunday as the first day, and many English calendars display Sunday first.

Is Friday named after Freyja or Frigg?

Etymology points to a Germanic love goddess parallel to Venus. Some scholars prefer Frigg (Odin’s wife), others Freyja; both match the Venus role. The exact attribution varies by dialect and historical evidence.

How do “On This Day” pages handle different weekday names?

They usually store dates with numeric weekday codes (ISO or iCalendar) and render names by locale. That prevents mismatches (e.g., English Thursday vs French jeudi vs Portuguese quinta‑feira) and avoids confusion around week-start conventions.