Many happy returns of the day. What does it mean
When we can use it? Many thanks in advance

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Just to refine Bob's answer slightly, it is ONLY used at birthdays. We would never use it in any other situation so it's not really a substitute for "congratulations" if someone has passed an exam, got engaged, or won the Republican nomination for president. 
It can also be shortened to "many happy returns"

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It's a British saying that means more or less "Congratulations".  It is often used instead of Happy Birthday.

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It is used in the United States. It's usually used for birthdays. The key is that it is a situation that deserves congratulations, but is expected to repeat annually.

For example, if your birthday falls on April 1st, then the day--April 1st--"returns" over and over again: April 1st, 2016; April 1st, 2017; April 1st, 2018. A wish for "many happy returns of the day" is thus a wish for a long life (many returns) and a happy life (many happy returns).

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