Groundhog Day date and weekday calculator by year

This calculator accepts a Gregorian calendar year and returns the complete ISO date, the English weekday name, and useful numeric weekday values. Its result comes from fixed calendar arithmetic rather than the computer clock, a time zone, or an online service. That makes it suitable for a quick personal lookup as well as repeatable software, publishing, event planning, and historical calendar workflows.

Enter the year you want to check

Provide one whole Gregorian calendar year between 1 and 9999. The calculator fixes the month and day as February 2 because Groundhog Day has the same calendar date every year, then determines which weekday contains that date. For example, entering a year is enough; you do not need to supply a location, time, or time zone. The result includes an ISO-formatted date with a four-digit year, the weekday written in English, an ISO weekday number where Monday is 1 and Sunday is 7, and a Sunday-origin number where Sunday is 0. Those explicit fields make the response easy to display or pass into another program. Text containing only digits is accepted by the underlying solver for compatibility with forms, while missing values, fractions, signs, words, and years outside the supported range are rejected. The calculation answers the calendar question only. It does not predict what a groundhog will see, report weather, or claim whether winter will continue.

How the weekday is calculated deterministically

The weekday is computed with integer Gregorian calendar arithmetic equivalent to Sakamoto's algorithm. February uses a fixed month offset, and the selected year is adjusted because January and February are treated as part of the previous arithmetic year. The calculation then accounts for ordinary four-year leap cycles, removes century years that are not leap years, and restores centuries divisible by 400. A final remainder produces a number from 0 through 6, which maps from Sunday through Saturday. The ISO weekday value is derived from the same remainder, so Monday becomes 1 and Sunday becomes 7. No JavaScript Date object is created. That matters because Date-based code can accidentally inherit a machine's local time zone, interpret early years inconsistently, or cross a day boundary when a UTC value is formatted locally. This implementation has no clock, network request, random value, locale-dependent parser, or mutable state. Identical validated input therefore produces identical JSON in the API worker and the free browser runner.

Use the result in calendars, content, and automation

The response is designed to serve both people and software. A content editor can verify the weekday before publishing an annual Groundhog Day article. A school or museum can label an activity schedule without manually paging through a calendar. An events application can use the ISO date as a stable key and the weekday name as display text, while analytical code can group results with either numeric weekday convention. Because the occasion always falls on February 2, the calculation is intentionally narrow and transparent: it does not add observance rules, substitute a weekday, or move the date when February 2 lands on a weekend. It also does not calculate regional festivals with similar names. For automated use, each request represents one year and costs $0.002; the browser version can be used interactively without an API request. Validate and store the returned ISO date rather than reconstructing it from prose, and choose the ISO weekday field when interoperating with systems that follow the Monday-through-Sunday international numbering convention.