About six years ago, I learned the Doomsday algorithm for computing dates, which relies on the fact that the last day of February, 4/4, 6/6, 8/8, 10/10, and 12/12 all fall on the same day of the week every year. Which day of the week this is is called the Doomsday for that year. Unfortunately, I do not have such a technique for easily recalling these Doomsdays, so I rely on a bit of calculation. My current strategy is as follows:
Use your left hand (or right hand if you’re like me and left-handed) and assign the fingers from Monday to Friday. Saturday and Sunday will be the gaps on either side - a “missing finger” if you will.
For 1900, start on Wednesday. For 2000, start on Tuesday.
Add 28 repeatedly - 28, 56, 84 (e.g. 1928, 1956…). You don’t move your finger during this calculation. Stop before you overflow (go past the year you want).
Add 4 repeatedly, moving back two fingers every time you do so. Again, stop before you overflow.
Add 1 for each year after that, moving forward one finger each time until you reach your desired year. The result will be the Doomsday.
For example, 1983. I would start on Wednesday for 1900, 1928, 1956, and move back twelve fingers until 1980, and move forward three for 1983. The result of the calculation is a Monday Doomsday, but it takes me about 5 seconds to compute this rapidly. From there, I have to calculate the month and day, which in a worst-case scenario is another 5 seconds. The finger I end up on is the result for that day.
Also, I don’t want to have to rely on a physical method for determining the day of the week. I wish I could do it mentally, and faster. (For calculating a calendar day this year for example - I still have to calculate the Doomsday for the year! 2000, 2004, 2008, 2012, 2016, 17, 18, 19… it’s Thursday. October 10 is Thursday, so calculate from there.)
I wonder if there’s an easier method. What am I doing that’s inefficient? Could I reduce the problem to a list of codes or a memory palace, placing all of the Tuesdays together or trying to link 19 (a roll of tape in my Major system) to 4 (Thursday - FOURSday, I start from 1 on Monday)?