The Doomsday rule#
The Doomsday rule is an algorithm that John Conway (from Conway's Game of Life) for determining what day of the week (Monday, Tuesday, etc.) a certain date is.
You can watch a video about it here
Doomsdays#
4/4,6/6,8/8,10/10,12/12,9/5,5/9,7/11,11/7 same day of the week which we call Doomsday Work 9 to 5 at the seven eleven January and febraury tricky due to leap year which is every 4 years. so if year % 4== 0 then it is a leap year 3/1 or 4/1 if leap year, 28/2 or 29/2 if leap year the only missing month now is march which you can remember with PI day 14/3
Other interesting doomsdays I will leave it up to u if u think these are coincidence? 31/10, Halloween 4/7, independence day, boxing day 26/12
So if you know pi day was on a monday then all these other doomsdays were also on a monday.
Any Date#
To now know which day a ceratain date was with the knowledge of knowing which day doomsday was all you need to do is add or subtract. so for example if u know doomsday in 2020 was a monday and you want to know what day christmas will be in 2020 all you need to do is subtract 1 as we know boxing day is a doomsday, or we can add 13 to 12/12, we can ignore 7s as monday + 7 days is monday again so we need to add 13-7=6 days to monday which is a sunday.
To find which day of the week Christmas Day of 2021 is, proceed as follows: in the year 2021, doomsday is on Sunday. Since December 12 is a doomsday, December 25, being thirteen days afterwards (two weeks less a day), fell on a Saturday. Christmas Day is always the day before doomsday.