This is a secret level of the game which can only be accessed once you've activated a couple of in-game triggers.
First of all you must steal the fly agaric from Dr. Bob's summer house, making sure not to damage any of the furniture and without waking up the crystalline guardian. Once you have it, you need to put the car on reverse during the suburbia scene, making you crash in the garage. This will bring you to Hades, where you must speak to the man with a lightbulb-shaped head who will give you an invitation to the thanksgiving dinner reception. Speaking to anyone else in the room will bring you to the next level which means you will have to start over.
Having done both of these things, you will finally be able to get to the level. In the level near the end of the game where you visit your childhood home, you have to give your mother the fly agaric while she is preparing the stew. It will bring you to the squid family's thanksgiving dinner, one of the most well-hidden secrets of the whole game. Congratulations!
You can walk around the house, though there isn't much interaction to be had. Be sure however to check out the bluegrass jam happening in the living room. Very cool.
On another note: Going to the dinner reception without having done the second step means you yourself will be the turkey, a most unfortunate fate.