What is the best time to trim out a lilac tree?

immediately after flowering

Start at any time that you can work outdoors (it is good to know your Zone)....trim first any broken, damaged, old, dead branches...then proceed to cut back as necessary....enjoy!!

Spring is a good time before all the leaves start showing up. There is usually a lot of dead wood in the center so you need to see what you are trimming. Open up the tree to light and air into the center.

Right after they finish blooming.

I would wait until the buds start coming out to see where the dead wood is, and wait until it is done flowering to trim back to shape or you probably won't have any flowers this year. It will also show you where it needs to be trimmed back, tie some thread or ribbon around the branches you want to eliminate.

Before it buds or after it blooms! If you prune while it it budding you won't have any blooms this year.

As several others have said, prune it after it is done flowering for the year. That will probably be some time in June or perhaps early July.

I always wait until fall to trim back my trees and shrubs....they will be going dormant and this gives them a time to rest.

Lilacs set buds for the next season's bloom shortly after the current season's bloom. Prune right after blooming to have blooms the following year. Tradition shrub trimming says to take out 1/3 of the old branches for three years in a row. However, with lilacs this old, which are a variety that are tougher than nails and need total rejuvenation, cut them down to 1 foot while they are dormant. You won't get blooms this year, but you will be on the way to a nice shrub in the future. I've even done this when they aren't dormant and haven't failed yet in 40 years. I also have newer varieties and most often prune in the fall after they drop their leaves. I once again took my dwarf Korean lilacs down to 1 foot last fall as they were 6 feet tall and overgrown. It works every time.

Bring in your plants

Serickson is correct.

I have a rather large lilac bush that's around 70 years old. I always trim and shape it as soon it quits blooming and the blooms all die. I never fail to have a beautiful full bush of blooms. Also I normally dig up and move it about it every 10 years. after I trim. I moved this bush from my Granpa's house in Virginia to mine in Ohio in 1969 after he passed away.I moved it from Ohio to NC in 1990 when i moved there then I moved it back to Ohio in 2006. When I pass it will be moved to my son's farm where it will have a permanent home

Hi Karen, you can prune it back. Just cut the branches you don't want, or those that are dead. They do like a good watering now and lots of sun. If your soil is on the dry side, you can amend it by adding Miracle-Gro® Garden Soil for Trees & Shrubs. In the spring, feed them with a good tree and shrub fertilizer. Here's a site that may add some more information: https://www.thespruce.com/lilac-bushes-growing-tips-2132722