The Washington Hilton was a happenin’ place to see live music apparently, with Hendrix also playing there in ’68. Thanksgiving was the following Thursday, so I imagine a large number of kids were home from college, and what better way to take a break than watch Morrison, Manzarek, Densmore and Krieger?
Thanks to Google, I found a number of these images here. And if you want to read a ridiculous amount of information about the Doors, go here. The Internet is amazing.