Literally: To be on your 31
Meaning: to get dressed up to the nines; to get dolled up;
Origins: unknown origins, different possibilities. Some attribute this expression to be an evolution of the word “trentain” which designated a superior quality fabric and the older expression “mettre sur soi” “ s’habiller”/ to dress.
Another possibility would come from a Jesuits celebration which takes place on July 31st (Ignace de Loyola celebration) and novices would receive new habits.
Another origin would come from the military world. Put your military uniform = tu mets ton 36. Other hypothetical military origin would be a reference to the end of months (31), which only occur 7 times a year, military privates would receive a special stipend to make month-end. On this occasion a complete review would be organized, and privates would have to clean up all their gears.
Synonyms: s’endimancher; se faire beau
Examples: je vais me mettre sur mon 31 pour rencontrer mes futurs beaux parents
I am going to dress up to meet with my future in-laws.