Flag on the play

Hi,
I'm translating "The Sopranos" for Swedish television viewers, and need an explanation of "flag on the play".
(A female psychiatrist has happened to double-book her clients, and when she enters the waiting room one of them says: "Flag on the play. I'm pretty sure you said Tuesday." I've tried the various on-line slang dictionaries, but had no luck.
Hilfe!

Niklas