The Rite of AshkEnte. When Galder Weatherwax first suggests it, instead of the "No, no, not that!" response he was expecting, the other wizards simply go, "Sure, why not?" Then when they summon Death, the reaper has a cocktail ("I was at a party."). Finally, after some exposition (During which Death refuses to go along with Weatherwax's attempts at drama), Death is allowed to leave;
Weatherwax: I hope it is a good party.
Death: At the moment it is. I think it might go downhill very quickly at midnight.
Death: That's when they think I'll be taking my mask off.
Funny by itself, but funnier if you know Edgar Allan Poe and The Masque of the Red Death.
Rincewind and Twoflower are fleeing Death's house while the Four Horsemen are distracted by an unusual card game that Twoflower has just taught them.
Rincewind: How long will that keep them busy?
Twoflower: I don't know. Probably until the last trump...