Francophonie· medium

se mettre sur son trente et un

to put oneself on one's thirty-one

Meaning

To dress in one's finest clothes

Explanation

For a big occasion, when only your best outfit will do.

Example

Pour le mariage, toute la famille s'est mise sur son trente et un.

For the wedding, the whole family dressed to the nines.

Origin

Probably a mangled memory of 'trentain', a fine medieval cloth woven from three thousand threads and worn only by the wealthy. It has nothing to do with New Year's Eve or the 31st of the month, despite what people often assume.

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