400-Year Calendar Loop and Friday the 13th
Our modern system, the Gregorian calendar, repeats its exact sequence of days, dates, and leap years every 400 years. This cycle consists of precisely 146,097 days, which divides perfectly into exactly 20,871 weeks. Because this loop is completely deterministic, the distribution of weekdays for any given date is set in stone and repeats indefinitely.
The roots of the rumor
The math and calendar magic
Global flavors of bad luck
The science of superstition
today This cycle of 146,097 days divides perfectly into 20,871 weeks, locking the weekday of every calendar date into a permanent loop.
How many years does it take for the Gregorian calendar cycle to completely repeat its sequence of days and dates?
In a single 400-year Gregorian calendar cycle, how many times does the 13th of the month fall on a Friday?
Which of the following weekdays is mathematically the least likely to host the 13th of the month?
Calendar Quirks: Due to leap year rules, the Gregorian calendar repeats perfectly every 400 years, meaning your future birthday weekdays are completely predetermined.
today Drag calendar months starting on Sunday to discover the secret behind Friday the thirteenth.
According to calendar arithmetic, what day of the week must a month start on to guarantee a Friday the 13th?
If October 1st falls on a Sunday, on what day of the week will October 13th of that year fall?
The day of the week after Saturday and before Monday, traditionally regarded as a day of rest. Every Friday the 13th requires the hosting month to open on this specific weekday.
today The Triplicate Years
Watch the calendar align this time (align this time!) / Three Friday the thirteenths in a triplicate year (oh yeah!)
What is the minimum number of Friday the 13ths that every single calendar year is guaranteed to have?
What is the maximum number of Friday the 13ths that can occur within a single calendar year?