Whatistheday.com is a free weekday calculator and multi-calendar date converter. It uses the Proleptic Gregorian Calendar for modern dates and the Julian Calendar for historical dates before the Gregorian Reform of 1582, ensuring 100% mathematical accuracy for any date from 4000 BC to the distant future. The tool supports over 10 calendar systems: Gregorian, Hebrew (Jewish), Islamic (Hijri), Julian, Proleptic Julian, Chinese Lunar, Hindu (Vikram Samvat), Maya (Long Count), Egyptian, and Babylonian. Users can toggle between BC/BCE and AD/CE eras to explore ancient dates with astronomical precision. The platform includes date calculators for business days, age, date difference, day of year, ISO week number, and leap year detection. It provides holiday calendars for over 50 countries, enabling cross-border planning and religious calendar synchronization. All calculations are performed client-side for instant results and complete privacy. Whatistheday serves historians, genealogists, educators, researchers, and professionals who need accurate date computations across cultures and time periods. The site is free, requires no registration, and works on any device.