Feb. 10, 2025
I bought a Barbara Hepworth calendar in the sale at the Tate Modern shop that I thought was a steal until I realised it was a 2024 calendar, not a 2025 one! However, the joke’s on them because I only have to wait until 2029 for the next year that starts with a Monday.
Unfortunately, unlike 2024, 2029 is not a leap year. This means all the days of the week would be wrong from March 1st onwards. It gets worse! The next two years that start on a Monday are 2035 and 2046, both of which quite obviously fail to be leap years.
Oh dear…
In fact, the next year I could use this calendar is 2080. Just as well that it has some nice pictures of Barbara Hepworth sculptures in it that I can look at while I am waiting!
In finding out just how poor a bargain my calendar was, I came across something called “Zeller’s Congruence”. This is an algorithm that helps you determine the day of the week for any Julian or Gregorian calendar date.
It looks like this for the Gregorian calendar:
\[ h = \left(q + \left\lfloor \frac{13(m+1)}{5} \right\rfloor + K + \left\lfloor \frac{K}{4} \right\rfloor + \left\lfloor \frac{J}{4} \right\rfloor + 5J \right) \mod 7 \]Where:
If, like me, you think that term for the month \( m \) looks a little bit weird, it’s because February is a pain in the backside and so the algorithm starts with March (scientifically proven to be the month of good omens) so that February can be considered last.