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I was born may 21 75, little sister oct 29 88, my oldest sister mar 12, 73. Why do our birthdays fall on the same day of the week every year?

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closed as off-topic by A E, McMagister, Tryth, xnor, Julian Rosen Jan 23 '15 at 6:22

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    $\begingroup$ This... isn't really a Puzzling question. $\endgroup$ – Pyraminx Jan 22 '15 at 5:44
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    $\begingroup$ I'm voting to close this question as off-topic because it's not a 'puzzle' in the sense that we use the word at this site. $\endgroup$ – A E Jan 22 '15 at 10:30
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The answer is simply because

The amount of days between those days of the year are divisible by 7 (the length of a week or a cycle of "days of the week"), so they will always land on the same day no matter the year. If one of those dates was before February of the same year then it'd change with leap years but that isn't the case.

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