Any shrub that flowers before the end of June is carrying the flowers on wood made the previous year. So the optimal time is immediately after the flowers fade.

Light shaping pruning can be done before the end of the growing season, but no more than 1/4 of that season's growth.

A good way of doing some of the pruning is to cut some of the stems before the flowers open. They will open in the house several weeks before the ones outside and you can then enjoy the flowers for a longer season! After flowering you can cut off all the flowered branches.

I agree with above, it's a little late to prune forsythia. Flower buds are already set for next spring. However a lite pruning will make little difference. Can you put it off until after it blooms next spring? I use July 4th as deadline to prune forsythia.