This traditional American summer classic is best made with very fresh peaches when they are in season in August and early September.
Be sure to buy them where they are grown! As with most fruit, the riper a peach gets, the softer it gets, so tree-ripe fruit is hard to pack and ship. Peaches shipped from Georgia or elsewhere are typically picked somewhat underripe, and then gassed with ethylene in a warehouse to "ripen" them artificially.