by Anura Guruge
Click to ENLARGE.
I don’t know if you ever thought about it but calendars (through their very nature) repeat themselves — i.e., you have years where the dates fall on the same days of the week (which means that January 1 on both years fell on the same day of the week). As the above image shows the calendar for 2016 is the same as that for 1988. So if you had a 1988 calendar you could reuse it, sans any issues, this year. For 2017 you could reuse that of 1989.
Basically, typically within OUR lifetime, calendars repeat every 28 years BUT this is NOT always the case. That has to do with the ‘NO LEAP DAY on century years that are NOT divisible by 400’ caveat. [That meant that there were NO February 29s in 1900, 1800 or 1700 since those century years are not divisible (exactly) by 400. But there was a February 29 in 2000, because it was divisible. Same in 1600. 2100, 2200 & 2300 will NOT contain Leap Days. 2400 will. Bit confusing … right?]
Once the 400 year rule is factored in our Gregorian Calendar (after the pope) TOTALLY repeats itself, without exceptions to do with Leap Years, every 400 years. During these 400 year cycles there will always be 97 Leap Years and as such 97 Leap Days.
15 of those 97 Leap Days will fall on a Monday, as is the case this year and WAS in 1988, 1960, 1932 & 1904.
BUT if you go back 28 years from 1904, i.e., to 1876, February 29 was on a Tuesday. That was because 1900 was a Century Year (i.e., divisible by 100) BUT was not divisible by 400.
15 Leap Days will also fall on a Wednesday. But only 13 will fall on a Sunday, Tuesday or Thursday, and 14 on Friday or Saturday. Isn’t that cool?
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by Anura Guruge