Leap Year

Why do certain calendars 'prevail' more often than others at certain times - for example April 9 falls on Tuesday and Sunday twice during an 11 year gap where Monday get skipped?



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Leap years cause a day to be skipped. If the 9th of April was on a Sunday in the year before a leap year, then Monday will be skipped in the leap year and it will be on a Tuesday. It then takes a number of years for Monday to come round again. That is what causes the 11 year gap.

A date happens on the same day either every 5, 6 or 11 years. Obviously there are 7 days in a week. But because of leap years, a day can be skipped, so it takes only 6 years for a day to come round again. It is also possible to have two leap years within a period, causing two days to be skipped and so we have a 5 year gap. But if a leap year skips a day, then it is an 11 year gap.

There is a pattern for how often a particular day happens on a day in terms of years. 6, 5, 6, 11. Then the pattern repeats. The only exception is of course the 29th of February. That usually repeats on the same day every 28 years. However, as years like 1900 and 2100 are not leap years, because they are divisible by 100 but not 400, the gap around them can be 12 years or 40 years, depending on how the dates fall.