The best time to visit Brentwood is late spring through early fall, timed around the farm trail season. Cherry picking typically starts in late April or May; stone fruits peak in June and July; corn and late summer crops carry on into September. If you are making the trip specifically for the farm trail, call ahead to farms to confirm what's currently in season and open for picking.
Cornfest, held in late July, is a lively if thoroughly local event — not a reason to make a special trip, but worth knowing about if you are in the area.
Winter is quiet and pleasant for a look around downtown, but there is little to draw you specifically to Brentwood outside of fruit season.