I would think that its best to book as soon as possible. Some areas are better than others for childcare so you may find it easy to find a place or it may be hard.
IMO it would be better to book to early and then have to postphone the starting date than to leave it too late and not be able to enrol in the center of your choice. It might be worthwhile to contact a few places and ask what thier waiting lists are and if you can have a look around. If you can visit a few places you will have a better idea about where you might want to enrol your baby. If all the babies look happy when you visit chances are yours will be too.