A worked example
January 1, 2000 — the dawn of the new millennium — fell on a Saturday.
Frequently asked questions
How does this figure out the day of the week?
It relies on the Gregorian calendar's well-defined structure — every date maps to exactly one day of the week, calculated from the total number of days elapsed since a known reference point.
Does this work for dates before standard calendar records, or far in the future?
It's accurate for any standard Gregorian calendar date, which covers the vast majority of historical and future date calculations you're likely to need.