Day of the Week Calculator

The day of the week calculator tells you which weekday any date falls on — Monday through Sunday — for any past or future date in the Gregorian calendar. It is a favourite for historical research, event planning, genealogy, and settling trivia arguments over "what day was that?"

What it shows

• Day name — Monday, Tuesday, and so on
• Day number — 1 (Sunday) through 7 (Saturday), matching the ISO convention used in scheduling systems
• Week number — the ISO 8601 week number within the year (week 1 is the week containing the first Thursday of the year)
• Leap year — whether the given year has 366 days, which also tells you whether February has a 29th

Fun historical dates

How it works

Determining the weekday algorithmically is a classic problem with several well-known solutions. The Gregorian calendar repeats in a 400-year cycle, and the weekday for any date can be derived from that cycle. Zeller's congruence and the Doomsday method are the two best-known formulations:

• Zeller's congruence converts year, month and day into a small modular arithmetic expression that yields the weekday directly, with careful handling of January and February (treated as months 13 and 14 of the previous year).
• The Doomsday method anchors every year to a memorable "anchor day" and then counts forward or backward by known monthly reference dates, making mental weekday calculation possible without a device.

JavaScript's built-in Date object performs the equivalent calendar arithmetic internally, accounting for leap years and the shifting pattern of days across years, which is what this calculator relies on. Because it uses the Gregorian rules, results are most accurate for dates from 1582 onward (when the Gregorian calendar was introduced) in regions that adopted it then; earlier or regionally-offset dates may follow the older Julian calendar.

Leap years and day shifts

The weekday of a fixed date advances by one place each ordinary year, and by two in a leap year. This is because a common year has 365 days (52 weeks plus 1 day), while a leap year has 366 (52 weeks plus 2 days). So if 15 August 2025 is a Friday, then 15 August 2026 will be a Saturday (a common year, +1), and 15 August 2024 — a leap year — was a Thursday (+2 back from 2025's Friday). You can use this single fact to check any near-year date by hand.

Uses

• Historical reconstruction — understanding diaries, newspapers and records that reference "the third Friday of" a month
• Event planning — choosing dates that avoid certain weekdays
• Scheduling rotations — recurring meetings, classes and shifts that fall on a weekday
• Trivia and documents — dates on certificates and photographs often need their weekday confirmed

Historical note

The seven-day week traces back to ancient Mesopotamia and the Babylonians, who assigned days to the seven classical planets; many languages still echo that naming (Sun-day, Moon-day, Saturn's day, and so on) to this day.