Find whether a date falls on Monday, Tuesday, and so on across Gregorian calendar years.
Overview
Scheduling contracts, historical research, and recurring events often need the weekday for a distant date. Mental math is error-prone across month lengths and leap years, so algorithms map calendar dates to a weekday index consistently.
Most implementations convert the date to a serial day count (Julian day number or similar), then apply modulo 7 with a fixed epoch anchor. Leap years adjust February length so March-based algorithms avoid February exceptions.
This calculator handles Gregorian rules: leap years divisible by 4 except century years unless divisible by 400. Historical calendars before adoption may differ, so verify era assumptions for genealogy work.
Use results to plan business-day offsets, verify birth certificates, or debug calendar software. Combine with a date-difference tool when you need both span length and endpoint weekdays.
Steps
Enter day, month, and four-digit year.
Assume Gregorian unless noted otherwise.
Run the modular weekday algorithm.
Review Monday–Sunday output.
Note if year is leap for February dates.
Increment one day to sanity-check sequence.
Math
Weekday index from serial day number modulo 7 with epoch offset
Date arithmetic converts Y-M-D to a day count, then maps remainder 0–6 to named weekdays per chosen convention.
Worked cases
Serial index mod 7
Tuesday.
Valid leap date in Gregorian calendar
Thursday.
1900 is not a Gregorian leap year
Thursday.
Why use this
Questions