Pruning should be done in the spring after flowering. This promotes the production of new wood. On this wood the flowers for next year will be formed. Forsythia can be pruned back fairly hard. Pruning to late will greatly decrease the number of flowers for the next year.
University of Vermont, USA
Notes & Reference
#01-Manual of Woody Landscape Plants (Michael Dirr), #226-Shrubs and Vines for American Gardens (Donald Wyman)