I usually keep an eye out for sales, checking out the clearance, sales bins, daily sales, etc. Lots of time sales come up with seasonal fabric designs a bit before the season and then just again after it. If you shop online it good to get emails from the shops you like. They usually keep you updated on good buys.
So basically, all year round is a good time. Just keep an eye out for the type of design or fabric you are looking for.