Lots of chickweed in re-seeded fescue lawn

I aerated and overseeded fescue the first week of September. A few weeks later I put out some more seed in the thinner areas. The original seeding has been mowed three times, but the newer grass is just now getting tall enough to mow.
I'd like to spray Weed B Gon Max on the very healthy chickweed crop, but don't want to hurt the newer grass.
How far into the fall (or winter) can I wait to try to kill the chickweed?
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