Seems like immediately after its done flowering is the recommended time. Also, it would appear that it flowers on old wood of 3+ years old.

If you're doing a massive pruning on a mature tree, it seems to be recommended to prune 1/3 of the branches each year for 3 years. Start with the oldest 1/3 one year, the middle 1/3 the next, and the youngest 1/3 on the third year.

I think that's to ensure that there's always at least some 3-year-old wood to allow flowering.

Normal pruning guidelines of removing crossing branches and such apply.

Caveat: I am not known for my pruning expertise.

Bob - you sound like you should be known for your pruning expertise. You know a lot more than I!

wow I wish my lilacs were as big as yours! if I had flowers weighing down the arrangements I would probably snip a whole bunch to put in vases around the house. i love the smell of lilacs!