The best time to buy bulk fruit for canning is when it's in season and readily available. Visit local farmers' markets, pick-your-own farms, or check with local orchards.  Typically, peak fruit seasons vary based on the specific fruit and your geographical location. Keep an eye out for harvest times in your area and take advantage of the abundance during those seasons.  And watch for end of season deals.  If an orchard has the last of it's fruit waiting, they want to get rid of it before it spoils.  You can often get seconds for a great deal.  Just remember fully ripe vs spoiled moldy is NOT the same thing.  Canning does not fix spoiled fruit.