You have to lay down the pre-emergent before the soil temp gets to 55 F. An old sage once told me to use certain Holidays to gauge one's lawn fertilization programme. Fertilizer with crab grass prevention around Valentine's day, apply a grass safe broadleaf weed control (pref liquid) around Easter if weeds are appearing, fertilize on or near Memorial Day and Labor Day, apply winterizer at Thanksgiving.
Don't know if the Old Sage knew WTH he was talking about, but it seemed to me a logical course of action to maintain and build one's turf.