The combination for me was 528, your code may be different. You find this safe by finding a Bone Charm in the sewers area in The Flooded District, in fact the Bone Charm is in the safe. Next to the safe you find a letter with a story in it.
There is also a secret in this area, here is a guide for it: Flooded District Sewers Area Secret: Helping Granny Rags versus Slackjaws
The months are important, remember the names and the order in which they are listed. next to the note is a book which has a list of months. The combination is the month’s position on the list, and the order is in the order in which the months were listen in the note.