Dishonored Guide: Opening the Safe in the Flooded District Sewers Area on the Way to the Hell Hound Pits Pub
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.