Here, in France we use to plant trees on this day, because there is a saying: on St Catherine's day " à la saint Catherine, tout bois prend racine"! on st Catherine's day if you plant a tree , it will take root.
But there is another custom, dating back to the Middle-Ages: A girl who was not married at 15 should wear a green and yellow hat during this day and should get married at Saint Nicolas(beginning of Decmber)
Nowadays, the young ladies who are not married at 25 were this hat and have a party on this day!
Before, the hat represented what they were doing as a work, with tools they used...Now they must be extravagant hats, but still in yellow and green!The yellow symbolises the faith and the passing time and the green youth and the hope of finding a husband!
Catherine wishes you a very good day!