When is the best time to prune lilac bushes?

Rule of themb for any flowering shrubs is after they have finished flowering.

http://homeguides.sfgate.com/time-trim-lilac-bush-47976.html

http://www.wikihow.com/Prune-Lilacs

I used to trim the wilted blossoms off after blooming. However, be careful how far DOWN (the wilted blossom) you cut off. I didn't know this fact (as a new gardener!) and went too far on the stem resulting in no blossoms the following Spring. Google this and you will see exactly where to cut on the stem. It won't hurt the shrub but you'll just have to wait for the second Spring to arrive.

Thank you Elaine!

Thank you Janet!!

Right after it blossoms...otherwise you cut off the new green shoots that blossom next year.

Prune them after blooming and flowers are dead. You'll see 2 leaves on the branch - cut above them. The pruning you do THIS year will be your flowers in two years. If you don't prune the dead ones this year, you'll only get a minimum of flowers the following year. Dana Ft Madison IA