Yeah, I book December when first out as I want specific flights. Now my return date has no lowest cost SW fares the entire day. So far, nothing cheaper. We always leave a few days before Christmas, as it gets closer to Christmas the fares creep up quite a bit.
I do think it is sometimes just plain luck to catch when an airline wants to fill a plane and starts by offering cheap seats.
And for me, with the lack of more than one or two SW direct flights, I book fast. Last year, my down flight reduced by around 20 dollars, I think in the summer too. Our dates are fixed due to school schedules, 20 to 31.