What Day of the Week Is It

From the proleptic Gregorian calendar — the one in use today, extended backwards past its introduction in 1582. A date recorded in the Julian calendar (Russia kept it until 1918) will therefore show a different weekday here than it did in the records of the time.

A common year is 52 weeks plus one day, so a fixed date moves one weekday forward every year, and two when a 29 February is passed. The whole cycle repeats after 28 years, except across century years that are not leap, such as 1900 and 2100.

Use the doomsday trick: within any one year the last day of February, 4 April, 6 June, 8 August, 10 October and 12 December all fall on the same weekday. For 2025 that day is Friday, so 6 June 2025 is a Friday and any nearby date is counted off from it in sevens.