Day of the Week Finder
Find which day of the week any date falls on using Zeller's congruence and Sakamoto's algorithm — plus day-of-year, ISO week, days until/since today, next/previous weekday finder, Nth weekday of month (e.g. 2nd Tuesday), recurring-weekday view across a year range, and Doomsday-rule teaching mode. 100% client-side.
About Day of the Week Finder
Find which day of the week any date falls on using Zeller's congruence and Sakamoto's algorithm — plus day-of-year, ISO week, days until/since today, next/previous weekday finder, Nth weekday of month (e.g. 2nd Tuesday), recurring-weekday view across a year range, and Doomsday-rule teaching mode. 100% client-side. Everything runs locally in your browser — your data never leaves your device.
How to use
- Enter your input in the tool above.
- Adjust any options to your preference.
- Use the Copy or Download buttons to save the result.
- Everything happens locally — your data never leaves your browser.
FAQ
What algorithms does the day-of-the-week finder use?
We compute the weekday two independent ways and check they agree: Zeller's congruence (a classic formula that maps year/month/day to a weekday number using integer floor division) and Sakamoto's algorithm (a compact 7-lookup table method). When both match the JavaScript Date object's result, you have three independent confirmations of the weekday.
How do I find the 2nd Tuesday or 4th Thursday of a month?
Use the 'Nth weekday of month' tab. Pick the year, month, the weekday you want, and which occurrence (1st–5th). The tool returns the exact date — perfect for figuring out US Thanksgiving (4th Thursday of November), Thanksgiving (2nd Monday of October in Canada), or any recurring monthly meeting.
What is the Doomsday rule and how does the teaching mode work?
The Doomsday rule (Conway's algorithm) is a mental-math shortcut: certain dates ('anchor days') always fall on the same weekday in any given year — e.g., 4/4, 6/6, 8/8, 10/10, 12/12, plus 5/9, 9/5, 7/11, 11/7 and Pi Day (3/14). The teaching mode walks you step-by-step: compute the century anchor, the year's doomsday, then walk from the nearest anchor date to your target.
Does this work for dates before the Gregorian calendar reform of 1582?
We use the proleptic Gregorian calendar (extending Gregorian rules backward) for all dates by default, which is standard for ISO 8601. The tool notes this and warns for pre-1582 historical dates where the Julian calendar was actually in use in most of Europe. Use the Julian-vs-Gregorian note for context on historical research.
What extra features does this tool have versus others?
(1) Zeller's congruence implementation. (2) Sakamoto's algorithm implementation. (3) Day-of-year (1–366) with leap-year handling. (4) ISO 8601 week number. (5) Days-until / days-since today. (6) Next/previous specific-weekday finder. (7) Nth weekday of month (1st–5th). (8) Recurring-weekday view across a year range (e.g., Christmas for the next 10 years). (9) Doomsday-rule step-by-step teaching mode. (10) Julian-vs-Gregorian historical note. (11) localStorage history (max 20). (12) Shareable URL. (13) Copy / download results as text.
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