Just to expand on Justfever's comment, Arashi calendar is where they celebrate lesser known holidays/days of celebration by doing a related activity~!
It's only a fairly new segment, since they revamped the show ^^ So far they've done things like skipping, testing who's voice is nicer, history of makeup in Japan, sport announcing, etc. It's hard to know what to expect because it's always a bit different v(^_^)v
I'm looking forward to VS Arashi so so so so much!!!!!
You're right though, so many Toma + Arashi collabs lately...
Maybe one day Arashi will do music station and Toma will just walk out with them like it's natural, hahaha