Any morning when one finds themselves coming down from a trip.
This definition comes from the phrase coined by Hunter S Thompson, "Every day is Saturday Night, Every morning, Sunday Morning".
This phrase also appears on the Ladytron song "the Weekend" and is referenced in several other songs by the same band.
Angie is falling asleep during first period on a Monday
Steve asks, "What's up with You?"
Angie replies, without lifting her head from the table, "Shut up, It's Sunday morning ><'."