## Zeller’s Congruence

### October 8, 2010

Zeller’s Congruence is a simple mathematical method for determining the day of the week for a given date.

In the 1880s, Christian Zeller, a German mathematician, noticed that, if you run a year from March through February, the cumulative number of days in each month forms a nearly straight line; that works because February, which would normally perturb the straight line, is moved to the end. He worked out the formula ⌊(13m−1)/5⌋ to give the number of weekdays that the start of the month moves each month, where m is the month number.

Then it is easy to calculate the day of the week for any given day: add the day of the month, the offset for the number of months since March, an offset for each year, and additional offsets for leap years and leap centuries (remembering to subtract one year for dates in January and February), taking the whole thing mod 7. It’s fun to work out the arithmetic yourself, but if you don’t want to take the time, the whole formula is shown in the solution.

Your task is to write a function that uses Zeller’s Congruence to calculate the day of the week for any given date. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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