Depends on where they are coming from, where they are going, what arrangements at each end, and pace of travel.
Once he retired, my brother made his moves from south-central Michigan to North-central Florida right after Labor Day, usually returning before the Memorial Day weekend. He would usually fly back to Michigan for Christmas, and sometimes for Thanksgiving. before retiring, his Florida season was shorter, particularly when he was doing mostly income tax planning work. He was sticks and bricks at both ends, representing the great majority of snowbirds.
My RV snowbird cousin who is Summer sticks and bricks in a northern Michigan forest cabin starts his move south in October, taking 2-4 weeks enroute, with some time in the mountains of the mid-south and on the upper Gulf Coast before settling into his South Florida RV resort. South Florida is way too hot for a Michigander if you get there too early. He makes no winter holiday trips back to Michigan because his children and grandchildren are in Florida. His return to northern Michigan usually starts mid to late April, arriving around the beginning of May.
I meet a lot of RV snowbirds who make this leisurely trip, leaving the North before it gets too cold (happens sooner in Alberta than in Arkansas) and following the autumn weather zone as it shifts to the south. I meet them because I go RVing locally in October and November, which is when those guys have stopped here, because it is too cold back home, still too hot in South Texas, and hurricane season is not yet over on the Gulf Coast.
A lot more sticks and bricks snowbirds, however, wait until after the winter holidays to make a rapid trip south, particularly if coming from places that don't get really cold until late December or January.
