I plan to go this year again which will make it my 3rd year. You can normally get the schedule of the week that always the most recent at http://imsaracing.net/#sebring
They have not released the provisional schedule and won't till I think late January. The week is like most racing weekends with support races and the like. The support series listed so far are, Patron GT3 Cup..aka Porsche Cup, IMSA Lites, and Star Mazda. They also normally have Skip Barber national series races also, which are insane. World Challenge has not posted their full schedule yet but I hope they return to sebring they have a good race there, and then there is the vintage groups.
SVRA sanctions the vintage race, and it is usally a large field of wide range of cars. I know not to long ago they had a GTP group and people loved it. Two years ago it was classic trans am.
For the week if I remember correclty it started off with private testing on the weekend. Monday and Tuesday were promoter and closed to public testing, this is when the line outside in the staging area builds up even more. To get on the test days you have to media or have a pass or an official.
I will include the link to last years, but wednesday was practice and qual for some series, Thursday had the Vintage race in the late afternoon then there was a break which was followed by the only Night session practice. Drivers who have not raced the track before a required to complete a minimum number of night laps prior to event. After this is followed by the traditional Green Park partying.....actually the entire weekend there is partying.
Friday is the support series races and final ALMS practice. Saturday is started off with an early race, either skippy or vintage. Then there is a break where cars are gridded and the fan grid walk happens. Everyone is allowed to participate in the grid walk, and you are actually on track and up close with the cars and flag girls. Then when that is over the race is on.