Find the day of the week for any date — past, present, or future
This calculator uses Tomohiko Sakamoto's algorithm — a compact and efficient method for determining the day of the week for any date in the Gregorian calendar. Along with the day of the week, it shows the day of the year, ISO week number, and days remaining in the year. The algorithm is based on Zeller's congruence but optimized for computational efficiency.
The calculator uses the Gregorian calendar, which was adopted at different times in different countries (1582 in some Catholic countries, 1752 in Britain and its colonies, later in others). For dates before a region's adoption of the Gregorian calendar, the result reflects the proleptic Gregorian calendar (extended backward) rather than the Julian calendar then in use.
The ISO week date system (ISO 8601) defines a week as starting on Monday and assigns week numbers 01 through 53. Week 01 is the week containing the first Thursday of the year. This is commonly used in business and finance for consistent weekly reporting.