How it works
Every date falls on one of seven weekdays, repeating in a fixed cycle. The calculator counts the whole days between your date and a reference date whose weekday is known (1 January 1970 was a Thursday), then takes the remainder after dividing by 7. That is the same method computers use, and it works for any date in the Gregorian calendar.
weekday = (days since 1 Jan 1970 + 4) mod 7 (0 = Sunday … 6 = Saturday)Worked example. 15 May 1990 is 7,439 days after 1 January 1970. (7,439 + 4) mod 7 = 7,443 mod 7 = 2, so it was a Tuesday — 20 Shawwal 1410 AH. Other famous dates: 1 January 2000 was a Saturday, and the first Moon landing, 20 July 1969, was a Sunday.
Why the weekday shifts each year
A common year has 365 days, which is 52 weeks and 1 day, so the same date moves one weekday later each year — two days after 29 February in a leap year. Your birthday in 2027 will be one weekday later than in 2026. The calendar pattern repeats every 28 years in most periods, and fully every 400 years.
Doomsday trick for mental calculation
Some dates always share a weekday in a given year: 4/4, 6/6, 8/8, 10/10, 12/12, 9/5, 5/9, 7/11, 11/7 and the last day of February. In 2026 these “doomsdays” are all Saturdays, so 10 October 2026 is a Saturday, and 15 October is the following Thursday.
To calculate your exact age, use the age calculator; for your age in Hijri years, the Hijri age calculator.