Depends on the store. I would say the best selection should be NOW, or earlier. I know this one boutique near me that always sells out of the good (most desirable) pieces early. Another had a large chunk of Prada's S/S line in December.. Nuts since they were trying to blow out F/W
I never buy anything full retail anymore though. No matter what it will always go on sale. S/S sales usually start around June, expect 10-25% off to start.