Day of the Week Calculator – Find What Day Any Date Falls On. Whatistheday.com is a free weekday calculator and multi-calendar date converter. It uses the Zeller’s Congruence algorithm to determine weekdays from date components directly, avoiding browser timezone shifts and lookup-table shortcuts. For modern and future dates, the calculator follows the Proleptic Gregorian Calendar. For historical context, it explains the Julian Calendar and the 1582 Gregorian Reform, when Catholic countries began correcting the drift between civil dates and the solar year. This matters because the same historical event may be recorded under different calendar systems depending on country, era, and source. The tool supports Gregorian, Hebrew, Islamic Hijri, Julian, Proleptic Julian, Chinese, Hindu Vikram Samvat, Maya Long Count, Egyptian, and Babylonian date views. BC and BCE dates are handled by converting between astronomical year numbering and traditional historical notation: astronomers use year 0 for 1 BC, but standard BC/AD writing has no year 0, so the interface clearly displays BC or AD while the math remains consistent internally. Users can pick a date, click GO TO, see the exact weekday, compare calendar conversions, and view an On This Day in History milestone for the same month and day. Whatistheday serves historians, genealogists, educators, researchers, students, writers, and professionals who need accurate date computations across cultures and time periods. The application is free, private, mobile-friendly, and performs calculations client-side. The homepage also explains frequently asked questions in plain language for crawlers and readers: how the weekday formula works, why calendar reforms matter, which world calendar systems are available, and how BC/BCE dates are displayed. This static source text helps AI search engines and traditional crawlers understand that the site is not merely a date picker, but a topical authority for weekday math, historical calendars, cross-calendar conversion, and date research methodology.