I would work on your budget a little bit before you go to Cabo. That way you will know how much you can allot to each item. You can start with things that the price WON'T change on based on season. So, write down how much you want to spend on your wedding dress, your invitations (don't forget postage), your photography budget. If you know you are having your wedding at dreams, then you can already account for the spa prices (hair, nails, etc.)
By adding all of these things up, you'll start to realize how much money you'll have for the wedding itself and for transportation and lodging. This could help narrow down dates for you.