You prune lilacs to get rid bad parts of the plant to maintain its health, keep it to a good and manageable size, and to encourage new growth, but knowing when to prune lilacs is important so that you really are helping your lilacs rather than hurting them. Here are a few key things to note about when to prune lilacs.
When your lilacs have reached six to eight feet tall and are fully matured is when they are ready to be pruned. Pruning before maturity can hurt the plant's chances of reaching maturation so be sure not to start pruning too early or you may damage your lilacs. Dwarf lilacs do not need as much pruning because they won't grow too large and out of control, but like all plants they will need pruning to get rid of dead and diseased branches and flowers.
It is best to prune your lilacs when the winter has finally passed and the freezing temperatures are no more. You generally prune at this time to get rid of dead and dying branches and flowers. Diseased and bug infested parts should be removed too so the plant's health is properly maintained. You also prune at this time so you could shape your lilacs so they are beautiful enough to display during the spring and summer. Prune during the spring to thin out dense areas with too many branches too so the flowers have enough room to grow and there is good air flow and enough sun entering the plant.
Another good time to prune is after the summer and well before the first winter frost so new buds have grown enough and are able to survive the cold weather. Between the end of the summer and early fall is when the buds usually grow, and you want to give them enough time to develop otherwise they may not survive the winter into the next year.