Not long ago, I fell into a rabbit hole on YouTube and found myself watching a video on how to determine what day of the week a particular date fell on. For example, you might hear November 22, 1963, and this technique helps you to quickly determine that it was a Friday.
For things like that, it’s admittedly of relatively little value. However, looking at just the current year it is much easier and likely more helpful.
What astonished me is learning how many days in any given year share the same day of the week. The idea is to remember those days, and what day of the week they represent, and it’s pretty easy to count from there. Some people call these “doomsdays” and others call them “pi day” (because March 14 is one of them).
The list starts like this, with these days all falling on the same day of the week:
- 4/4, 6/6, 8/8, 10/10 and 12/12. That worked out great!
- Next, imagine that you need to work 9-5 at 7/11. Well, we also get 9/5, 5/9, 7/11, and 11/7.
- Taking it further, this day of the week is also the same on pi day (3/14), Independence Day (7/4), Halloween (10/31) and Boxing Day (12/26).
Very quickly we know 13 days of the year that always fall on the same day of the week as one another.
Leap years…
Notice that we haven’t talked about January or February yet, because leap years can mess those up. Well, leap years don’t occur 3 out of 4 years, so Jan 3 is one, as is the last day of Feb (2/28).
If it’s a leap year, which occurs one out of every 4 years, Jan 4 is one as well as the new last day of Feb (2/29).
We have the list!
I know that was a lot to follow, but that is by far the most difficult part. Now we know that every one of these dates falls on the same day of the week in a given year:
- Jan 3 or 4
- Feb 28 or 29
- Mar 14 (3/14, pi day)
- Apr 4 (4/4)
- May 9 (5/9)
- Jun 6 (6/6)
- Jul 4 (Independence Day) and 11 (7/11)
- Aug 8 (8/8)
- Sep 5 (9/5)
- Oct 10 (10/10) and 31 (Halloween)
- Nov 7 (11/7)
- Dec 12 (12/12) and 26 (boxing day)
In 2023, that magic day will be a Tuesday. If you can remember that, and work on the list above, it’s super easy to know the day of the week for any given date.
- When is Christmas? Well, we know that Boxing Day is a Tuesday, it’ll be on Monday.
- When is Halloween? It’s a magic date, so we immediately know it’s a Tuesday.
- How about St. Patrick’s Day on March 17? We know that March 14 (pi day) is a Tuesday, so just count three from there and we know that St. Patrick’s Day is a Friday next year.
What year?
Now that you understand those dates within a given year, you need to know the “pi day” for any other year. Once you know that, you can quickly get any date in any year. For example, I’m writing this post in 2022 and our “pi day” this year was a Monday. All of those dates above are Monday, so you can do the math on any date from there.
Working out older years is a little tricky, but isn’t too bad. At the same time, learning this for past years is much less useful and really just more of a party trick. Either way, this short video is a great way to understand how it works:
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