Hope everyone is having a safe and lucky Friday the 13th. We’re enjoying it and preparing for Valentine’s Day tomorrow.
I looked up Friday the 13th on Grokipedia and found this near the bottom of the article. It is a very nerdy description of how to calculate the date without looking at a calendar. Enjoy, if you’re a nerd like me.
Yearly Variations in Occurrence
In the Gregorian calendar, the number of Friday the 13ths occurring in a single year varies between one and three, with no year featuring zero or four such dates. This limitation arises from the calendar’s structure, which consists of 365 or 366 days distributed across 12 months, resulting in exactly 13 occurrences of the 13th across all months but constrained by the seven-day week cycle to produce at most three Fridays among them.[83]
Years with three Friday the 13ths typically follow specific patterns based on the starting day of the year and whether it is a common or leap year. In common years beginning on a Thursday, the dates fall in February, March, and November, as seen in 2015. Similarly, 2026, a common year starting on a Thursday, will have Friday the 13ths in February, March, and November. These configurations highlight how the alignment of the year’s first day influences the distribution, with February, March, and November forming a common triplet due to the cumulative day offsets in non-leap years.[83]
To predict the exact day of the week for any 13th, including Fridays, Zeller’s congruence provides an algorithmic method tailored to the Gregorian calendar. Devised by Christian Zeller in the 19th century, the formula calculates the weekday as follows:
Where,
- h represents the day of the week (0 for Saturday, 1 for Sunday, …, 6 for Friday);
- q is the day of the month (13);
- m is the month (March = 3, April = 4, …, December = 12, with January and February treated as months 13 and 14 of the preceding year);
- K is the year of the century (year mod 100)
- and J is the century (|year/100|)
- A result of h=6 (mod 7) confirms a Friday.
This congruence enables precise determination of Friday the 13ths for any year by applying it to each month’s 13th, revealing the yearly variations without manual calendar inspection.
Since we’re probably not going to post a blog tomorrow, please enjoy Valentine’s Day with your special ones.
Saturday, 02/14/26 VALENTINE’S DAY: I took this photo of a heart-shaped tree mobile this morning . . .
