Don't read the question if you haven't finish the volume 4 yet.
Spoiler! :
Did Kyon really came back to his original world? If you think about it, the original plan for SOS's Christmas party was a hotpot. Then when he came back, it became some charity work... I have a feeling something is not complete here.
the meeting about the charity work was on the 18th which was the day everything went haywire
in other words hotpot plan for chrismas eve
charity work christmas day maybe?