I cannot comment first-hand on Miami. But I did get some info recently on the general trends from the communications person at a major U.S. airport.
She said (parenthetical comments mine):
The volume on given dates varies each year depending on the day of the week on which Christmas occurs.
In 2005, Christmas was on a Sunday, so travel concentrated into the Friday, Monday and Tuesday "bookends" of the long weekend. When the holiday is mid-week, travel gets spread out more. (So by traveling Thursday 22.12 last year, you missed Friday's chaos.) The 2 or 3 days after Christmas can be especially difficult because you have a combination of travelers returning home from holiday visits and people beginning trips after Holidaying at home.
This year, Christmas is on Monday. While that will concentrate a lot of travel into Friday 22.12, some % of folks will find it satisfactory to do their over-the-river-and-through-the-woods flying on Saturday the 23rd. Tuesday the 26th and Wednesday the 27th will be rough - usually the post-holiday travel spreads over three days, but look for more concentration in those two days this year because of Christmas being Monday.
Airports and the FAA will post news releases in Oct/Nov trying to steer people away from the period 26.12 to 28.12.
(Hmm, Thursday 21.12 is starting to look attractive.)
(Funny, none of the FAA or airport information mentions any worries about travel on Christmas even or Christmas day. I'd have thought Christmas eve would be a rough day. Then again, this year it is a Sunday.)
Finally, she said, avoid January 2.