Best time to divide a Lilac

We have two lilacs in the front of our house with a number of shoots we want to separate and replant.
When is the best time to divide? Before they bloom, right after the bloom, in the fall?
Also, if there are any we don't want to replant, will removing them help the main lilac or should we just leave them?
We live in the northeast in Zone 5.
Thanks, rw
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before they leaf out, after they dropped their leaves. Ingrid

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