It's about timing. If you prune right after it blooms there is time for new growth to form for blooms next year. If you wait until winter to prune, you will prune off all your last years growth, so no blooms. Lilac blooms on last years wood. OK?

Millie said it well but I would add to prune out any dead or very old wood but not too much and aren't you supposed to prune off the old blooms too?Posy Pet

Posy Pet is right about old, or dead wood...I was just thinking "small plant", but you will need to do that some time in the future. Does anyone else remember when they used to hide the little house out back with a huge old lilac bush? ;O)

I do not think that was to hide it. No smelly sprays to cover the deposits so had the fine something that did not mind sticking around to cover up the smell.