Tukuk-OS📅 Weekday Finder
ℹ️ What this tool does & how it works
One call to Date.UTC (which never touches your local timezone) gives the real weekday for any civil date. From there this page does the two party tricks: scanning years to find when a fixed date lands on a chosen weekday, and counting N days forward or backward while carrying the answer in both date and weekday form.
How a weekday is computed at all
Under the surface this is Zeller's congruence: take the day, add the month and year offsets from a table, add the leap corrections, reduce modulo 7. The browser's Date.UTC(...).getUTCDay() is that arithmetic wearing a nicer hat, and using the UTC constructor matters — the local-time version can silently shift a date by a day across daylight-saving boundaries, which is exactly the bug that makes "what day is my birthday" apps wrong for some users. The ISO week number follows the Thursday rule: the week containing January 4 is always week 1.