There are actually several lines which refer to the holiday-like atmosphere of the Rex Manning signing at the Empire Records store. The first is between Mark, the typical stoner employee, and Joe, the manager, as they open the store for the day.
Mark: What's up with the boss threads, man?
Joe:It's friggin' Rex Manning Day.
Mark: What's up with the hosility, Joe?
Mark and A.J., the art school student employee, begin singing Rex Manning lyrics as they enter the building
The next reference is a scene later, when Gina picks up Corey at her house, and Corey brings her a cupcake.
Corey: Surprise! Happy Rex Manning Day!
Gina: When did you have time to make these?
Corey: My dad always says there are twenty-four usable hours in every day, thank you.
Gina: It is official, you are a nerd.
The next lines are between Joe and Corey. Corey is working on homework in the back of the store, and Joe informs her she isn't scheduled until the afternoon.
Joe: Corey, what are you doing here?
Corey: Calculus. I hate it, but my dad says I've got to get an A.
Joe: No, I mean, you know you're not on until this afternoon?
Corey: Joe, it's Rex Manning Day.
Joe:(very slowly) Rex Manning Day.
The last actual reference to Rex Manning Day is said by Mark, just before Video Killed the Radio Star begins playing.
Mark: But we can't be sad. Not on Rex Manning Day!
cue the music
On a concluding note, Empire Records is an excellent film, full of great songs and great dialogue. It's sad it didn't make as big a splash with the mainstream movie goers as it did with the underground clan. But we will worship it forever.
Damn the man. Save the Empire!