When is it the best time to trim a lilac bush.?

You can trim it right after it is done blooming. Don't wait too long. Don't trim any more that a third of the plant.

AFTER it has bloomed. You'll have an idea of which part is too big, or dense, or flowerless... so wait a bit.

ALSO---DO NOT remove more than 1/3 of any shrub you want to keep... it may not recover.

immediately after flowering has finished

After it blooms and BEFORE the 4th of July. Pruning like this will bring more blooms every year. Don't prune more than 1/3 of the total bush per year.

Trim it back (no more than 1/3 of the bush in any given year) after it has finished blooming.

Right after it's done blooming.

Thank you all for the answers!

Linda, one more. If they are too tall or awkward or out of shape, cut down to the ground and let the new growth replace it. Keep those in shape. Lilacs are very hardy. We cut all ours down once and have them back again. If they get too tall you can't cut the flowers.

After it completed it's blooming cycle

Hi Linda, here's a great article and video that will show you how and when to prune your lilac bush https://www.thespruce.com/pruning-lilacs-how-and-when-to-prune-lilacs-1403004