Problem #WSP-000253

Problems Number Theory Divisibility Division with remainders. Arithmetic of remainders Division with remainder

Problem

You might want to know what day of the week your birthday is this year. Mathematician John Conway invented an algorithm called the ‘Doomsday Rule’ to determine which day of the week a particular date falls on. It works by finding the ‘anchor day’ for the year that you’re working in. For 2025, the anchor day is Friday. Certain days in the calendar always fall on the anchor day. Some memorable ones are the following:
0’ of March - which is 29th February in a leap year, and 28th February otherwise.

4th April, 6th June, 8th August, 10th October and 12th December. These are easier to remember as 4/4, 6/6, 8/8, 10/10 and 12/12.

9th May, 11th July, 5th September and 7th November. These are easier to see as 9/5, 11/7, 5/9 and 7/11. A mnemonic for them is “9-5 at the 7-11".
Then find the nearest one of these dates to the date that you’re looking for and find remainders.

For example, π day, (14th March, which is written 3/14 in American date notation. It’s also Albert Einstein’s birthday) is exactly 14 days after ‘0’th March, so is the same day of the week - Friday in 2025.

What day of the week will 25th December be in 2025?