To those who still don't get the joke. This is part of some quick sketches where Momiji appears to be celebrating the fact that it's "Friday", and it is commonly known that Friday is a great day. So in this illustration, Momiji seems to be beating Monday so that it changes to Friday or the weekend.
Updated
Monday
(Note: Although it is the kanji of "Month" and "Moon", in this context it is referring to Monday (月曜日). )
Until you become a day off, I won't stop beating the hell outta you!!