Weekday Finder

This tool finds the day of the week for any past or future date, and also calculates the date and weekday N days after or before a reference date — handy for planning appointments, anniversaries, and deadlines.

The input date is normalized to UTC midnight, then JavaScript's Date.getDay() returns a weekday index (0=Sunday to 6=Saturday), which is mapped to its name.

Weekdays repeat every 7 days, so if you know the weekday of just one reference date, you can find any other date's weekday from the remainder of the day-count difference divided by 7. Zeller's congruence, the classic historical formula, is really the same remainder logic broken down by month, day, and century.

For example, January 1, 2026 is a Thursday. There are 197 days until July 17, 2026, and 197 ÷ 7 = 28 weeks with a remainder of 1 — shifting one day past Thursday lands on Friday, which matches the actual result. This calculator applies the same idea internally: JavaScript's Date object counts elapsed days from January 1, 1970 (a Thursday) and takes the remainder mod 7.

A year isn't exactly 52 weeks — it's 52 weeks + 1 day in a common year (365 days) or 52 weeks + 2 days in a leap year (366 days). So a fixed date like a birthday shifts by one weekday across a common year, or two weekdays across a year that includes February 29.

In practice, January 1, 2023 was a Sunday, and January 1, 2024 was a Monday — a one-day shift, since 2023 was a common year. But January 1, 2025 was a Wednesday, a two-day shift, because 2024 was a leap year with 366 days. 2025 was a common year again, so January 1, 2026 shifted only one day to Thursday.