The earlier you can boo your neighbors, the better. That gives them enough time to put together gifts for other neighbors and for the chain to carry itself throughout the neighborhood. Personally, I wait until the first full week of October to boo our neighbors. Anything earlier than that feels too early. Also, the "We've Been Boo'd" sign is meant to hang on a neighbors door after they've been booed and until Halloween, so no one really wants that on their door for a full month.