Are you going to the Symphony or to the Opera tonight? Getting dressed up for a nice occasion? In that case, you can say: Ce soir, je me mets sur mon 31! (“Tonight, I’m dressing up”).
The expression se mettre sur son 31, literally “to put on one’s thirty one”, dates back to the 19th century. Its origin is not well known. However, one explanation seems to stand out. The word 31 (trente-et-un) is probably a distortion of trentain. The trentain was a very luxurious fabric used only for very special occasions by the high society. It was not well-known by a large part of the population and for this reason might have been pronounced trente-et-un instead of trentain.