This is an activity at the hackerspace. It's name is derived from the German word for a mop: "Wischmop".
From this description you can already conclude that it has something to do with cleaning the space.
Wishmobbing is the participation in a Wishmob. This is a sponaneous action (like a flashmob) triggered by unbearable dirtyness of various spots and a general subliminal desire of other members that "someone should really clean here a bit".
It usually results in a couple of people stopping their current activity and engaging in cleaning. The distribution of nice smells throughout the building usually bring other members to join in and in a couple of minutes a lot of busy bees are sucking the floor, cleaning the toilests, sweep the chill and doing the dishes.
A wishmob is over as fast as it came and results in general well being of the fellow hackers.
It should not be mistaken with the CleanSpace_Days despite the result being similar.