If a certain number of photos taken in the previous month have been uploaded, stickers with the current month's design will be delivered around the third day of each month. (Depending on when you subscribe, the designs you receive may be different from the designs offered when you subscribed.)
Please see I'm not receiving sticker suggestions. What should I do? for more information on the kind of photos that can be made into stickers.
If sufficient photos taken in the previous month have not been uploaded by the time stickers are being generated, past uploads will be used instead.