Starts at 3 PM and ends at 11 PM. Some of Season 6 along with 7, and it looks like there are quite a bit of notable episodes thrown in there.

Notable episodes:

Season 6 Episode 20 - My Son, The Driver (Brad refuses to tell the truth about his car accident, three generations of Al Unser car racing family visit Tool Time)

The first marathon episode which starts at 3.

Season 6 Episode 23

The Feminine Mistake

I believe this one was originally a 3D episode if I'm not mistaken

Season 6 Episode 24

Taps

Jill endures a family tragedy. Polly Holliday guest stars

Other notable episodes in the marathon:

Season 6 Episode 25, which is the episode where Pamela Anderson returns for a guest role

Season 7 Episode 5, which happens to be the Mark's horror film one

Season 7 Episode 9, which is another Thanksgiving episode and the family celebrate it at a Detroit Lions football game. Has guest stars such as Rodney Dangerfield along with some others.

There's also the usual marathon later on today, all episodes are from Season 6.

One notable episode on today:

4:00-4:30 PM, which happens to be The Beach Boys episode and Tim is surprised to learn that Wilson's cousins are none other than the band members themselves.