By using this mental trick developed by mathematician John H. Conway, you can figure out the day of the week for any date, past or future, in a few seconds with just one hand. And no, that one hand is not looking to Google for the answer.
It uses an algorithm he devised called the Doomsday Rule:
[It draws] inspiration from Lewis Carroll's perpetual calendar algorithm. It takes advantage of each year having a certain day of the week, called the doomsday, upon which certain easy-to-remember dates fall; for example, 4/4, 6/6, 8/8, 10/10, 12/12, and the last day of February all occur on the same day of the week in any year. Applying the Doomsday algorithm involves three steps:
Determination of the anchor day for the century.
Calculation of the doomsday for the year from the anchor day.
Selection of the closest date out of those that always fall on the doomsday, e.g., 4/4 and 6/6, and count of the number of days (modulo 7) between that date and the date in question to arrive at the day of the week.
This technique applies to both the Gregorian calendar A.D. and the Julian calendar, although their doomsdays are usually different days of the week.
Here's the video's "cheatsheet" if you want to give it a go.
