Mental calculation of the day of birth

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I have a friend who can tell the birth day of anyone by only knowing the date and year. He is able to do this almost instantaneously. How does he do it? Please enlighten me on this subject. Thanks a lot in advance.

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It's the doomsday rule.Hats off to its creator-John Horton Conway.

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More information is given in this page-https://en.wikipedia.org/wiki/Doomsday_rule .