Answer: Because lilacs develop next year's flower buds immediately after flowering finishes, the best time to prune is as soon as the flowers are spent. If you prune later in the summer, you may be pruning off next spring's flower display. If that doesn't matter to you, you can prune any time during the summer. If you're transplanting a lilac, it isn't necessary to prune it back at planting. In fact, the act of being transplanted will be enough of a shock to your lilac and pruning will just add insult to injury. I'd plant it, give it an entire season or more to become established and then prune it in late spring after it has flowered. If it doesn't flower on schedule, prune it in late spring, just as if it had flowered. Pruning at that time will encourage new growth which (hopefully) will include the development of flower buds for next year's display.
Best wishes with your lilac!
Q&A Library Searching Tips