When is the right time of year to prune lilac bushes

You trim Lilacs right after they bloom....next years flowers are on this years growth....trim to late....no flowers next year

Immediately after they have finished flowering.

The above 2 are correct; good time to trim also.

Look for any diseased or damaged branches. Take those out. Look for branches that are too close or rubbing against each other in the wind. Take the weaker one out. Shape it into a pleasing look and don't let it outgrow its intended area. Mine were planted, long before I bought the place, underneath the porch window. If I want to see outside I have to do vigorous pruning at least every other year.