Calendar Calculations with Cards

As readers of this previous post will know, I’m rather fond of mental calendar calculations. My friend Al Stanger, with whom I share a passion for recreational mathematics, came up with a remarkable procedure for finding the day of the week corresponding to any date in history using just a handful of playing cards. What’s particularly noteworthy about Al’s algorithm is that it involves no calculations whatsoever, and the information which needs to be looked up can be cleanly displayed on one of the cards.

When you work through Al’s procedure, it will feel like you’re performing a card trick on yourself – you will be amazed, surprised, and will likely have no idea how it works. I’ve never seen anything quite like this before, and I’m grateful to Al for allowing me to share his discovery with the public for the first time here on this blog.

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A Very Meta Monday

Usually my blog posts are rather tightly focused, but today I’d just like to post a few stream-of-consciousness thoughts.

(1) My blog was recently featured in the AMS Blog on Math Blogs. Perhaps by mentioning this here I can create some sort of infinite recursion which crashes the internet and forces a reboot of the year 2020.

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Mental Math and Calendar Calculations

In this previous post, I recalled a discussion I once had with John Conway about the pros and cons of different systems for mentally calculating the day of the week for any given date. In this post, I’ll present two of the most popular systems for doing this, the “Apocryphal Method” [Note added 5/3/20: In a previous version of this post I called this the Gauss-Zeller algorithm, but its roots go back even further than Gauss] and Conway’s Doomsday Method. I personally use a modified verison of the apocryphal method. I’ll present both systems in a way which allows for a direct comparison of their relative merits and let you, dear reader, decide for yourself which one to learn.

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