8DAYS

In the 2nd level of the game (labs) continue through the sections until you get to a large area with lots of robot sensors and several locker room/rest rooms areas near the top of the screen. On the far rightmost locker room, you will see an 'X' appear when standing in front of the lockers (this isn't typical for lockers).

Press the 'X' button, and it will open a hidden door. Enter the room and follow it down into a room with a disco ball and a bunch of people dancing. Achievement unlocked.

Sorry if the location description is a little confusing, but if you walk up to all lockers as you play through the level, you should be able to spot it fairly easily.

NOTE: FYI, "So long and thanks for all the fish" is a reference to The Hitchhiker's Guide to the Galaxy