Burt Kaliski Jr. (@modulomathy)
4/1/12 10:35 PM
#math trivia for #April1: #92 = 7*13+1, so April is congruent to January; both start on Sunday. What other months are congruent this year?
The answer is a simple matter of modular arithmetic. Reduced modulo 7, the lengths of the 12 months in a leap year follow the sequence
3 1 3 2 3 2 3 3 2 3 2 3
The starting days of the month thus have the modular values (relative to January 1)
0 3 4 0 2 5 0 3 6 1 4 6
The congruent months this year are
— January, April and July
— February and August
— March and November
— September and December
January 13 was a Friday. Due to the congruences, there was also be a Friday the 13th this year in April, and a third one, the maximum for any year, in July.
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