I feel like we've missed something... I'd probably have to read Haruhi Suzumiya-chan to get it though.

Yes, another Haruhi-chan gag. Everyone dresses up to scare Kyon (except Mikuru who dresses cute... because she's Mikuru). It's also blisteringly hot, so they basically become delirious while waiting for him to show up.

Halloween costumes. Asahina went as Sheep-san, and Haruhi went as a cyclops -- to be honest I thought it was a shoddily done Friend mask, but that wasn't the case -- anyways, they just overheated, or something.

Grahf said:
to be honest I thought it was a shoddily done Friend mask

That's funny, when I saw this image that was the first thing I thought of too.