Dominical letter

The dominical letter is a letter or two associated with each year that tells one which days of the year are Sundays. January 1 to 7 are always considered A to G, after which the cycle repeats to the end of the year (ignoring February 29 in leap years). Common years have one dominical letter, while leap years have two. For example, the dominical letter of the current year (2024) is.

Julian calendar
The Julian calendar repeats every 28 years.

Gregorian calendar
The Gregorian calendar repeats every 400 years.