These are spring flowering plants. Don't prune spring flowering plants until after they bloom, because the flower buds were formed last year.

You can prune dieback anytime though. And if you are happy with the appearance of the plant, don't feel you need to prune any live growth.

Shazam is right. A good rule for pruning any flowering shrubs bushes or trees is to do it soon after they are finished flowering unless they are fruit trees.

Great advice, thanks, I will prune the lilac after it flowers.
Bought cherry tree mid summer last year, 5 cherries came with the package on the tree, ended up in bird poop before I got hold on them...
Kinda hope I will get a bit more this year ... breathing in, breathing out ...
Rick

Rick- like the rest said if you're pleased with the shape of your lilac tree don't prune it.HOWEVER you must remove the spent flower panicle in order for you to have blooms the following year, if you don't the flower production will decrease greatly.If the lilac is of a suckering form remove those you don't want at ground level.
Lois