‘Tis the season for New Year’s resolutions. As we all sign up for gym memberships or invest in a Peloton that will just gather dust by March, I resolve to work out your brain this year. January 1, 2024 is a Monday. So what day was January 1 last year? Because of the regularities in how we slice and organize time, you don’t need to consult a calendar or count backwards from 365 to answer that question. Legendary mathematician John Conway, most famous for his , exploited the structure in calendars to develop an algorithm that computes the day of the week for any date, past or future. It’s a little too complicated to lay out here, but I think with an afternoon’s worth of practice any of you could learn to do it in your head. If you want to impress your friends with your Rain Man-esque skills, read about the and please report back. Whether or not you choose to become a Time Lord, this week’s challenge will test your understanding of how we organize time. Happy new year, fellow puzzle-lovers. You sell calendars without years written on them. If somebody wants a 2024 calendar, you give them one whose days of the week match up correctly with 2024 (January 1 is a Monday, January 2 is a Tuesday, etc.) The last question is particularly tricky, and you may need to remind yourself how the works! I’ll be back next week with the solution and a new puzzle. Do you know a cool puzzle that I should feature here? Message me on Twitter or email me at

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