Day of Week Calculator

This day of the week calculator tells you what weekday any date falls on—past, present, or future. It is designed for anyone curious about the weekday of a birthday, historical event, or upcoming date, and for students learning calendar algorithms. The tool outputs the weekday in large text, the full formatted date, a day-of-year position (for example, day 48 of 365), which occurrence of that weekday it is in the year, and a rotating trivia fact. The Math tab shows a step-by-step Doomsday Algorithm proof for your selected date, including the century anchor and year formula. It does not calculate date differences, add or subtract days, or count business days—for those tasks, use the date calculator or day counter. Results update live as you change the date.

Pick a date in the date picker; the result updates as you type. The main output shows the weekday in large type (e.g., "It's a Tuesday!") with the full long-form date below. A "Did you know?" line rotates facts about that day. "Go to Today" jumps back to the current date. No Calculate or Reset, everything updates live.

The Doomsday Rule uses modular arithmetic to find the weekday of any date. Each year has a "Doomsday", a weekday that easy dates (4/4, 6/6, 8/8, 12/12) fall on. The formula $$f = (d + a + b + c) \bmod 7$$ combines the century anchor d (e.g., Tuesday for 2000s) with year terms a = ⌊yy/12⌋, b = yy mod 12, c = ⌊b/4⌋. From there, count forward or back from the nearest doomsday. The Math tab shows the full proof for your selected date.

The result card shows where your date sits in the year: day number out of 365 (or 366), days left in the year, and which occurrence of that weekday it is (e.g., the 7th Tuesday of 52). Useful for project milestones, countdowns, and calendar trivia.

Most English day names come from the seven "wandering stars" visible in antiquity, the Sun, Moon, and five planets. Sunday and Monday are straightforward. Tuesday through Friday honor Norse gods (Tiw, Woden, Thor, Frigg), mirroring Roman gods (Mars, Mercury, Jupiter, Venus). Saturday kept its Roman name. Saturn's day. "Thursday" nods to Thor; "Wednesday" to Woden (Odin). The Day Trivia tab goes deeper.