When is the best time to replant?

i live in new jersey & would like to move lilac bushes and a erfly bush to my new home. when can i do that? i know i have to prune them back severly first ( they're real big ) and dig a big deep root ball to get all the root but are there any other instructions i need to know before i start? if i don't get around to doing it this season when's the best time next season?