Je dois me mettre sur mon trente-et-un.

English Translation

I have to dress up.

se mettre sur son trente et un = to get all dressed up
être sur son trente et un = to be dressed up to the nines
© Larousse 2014

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trente-et-un because that’s the date for new years eve (12/31)

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That will help me to remember it, thanks for that.

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