I'd disagree that Thanksgiving day is the heaviest air travel day of the year. More likely flights the day/days before are impacted. Quick Google searches are indicating both the Monday and Wednesday flights are busy this year.

Also think that few folks also want to get up before o'dark thirty on Thanksgiving anyway so a 6:15 am departure out of BWI should be not super busy. Given UA's lack of strength there, probably would want to look at DL and F9's early departure schedules to see who might be joining her in the TSA lines.

David