Presumably, it's the Russian equivalent of last-day-of-school pranks. In my sister's year, a sizable minority cross-dressed for the day, while another group hid eggs around the school. My year group celebrated with water fights and bubbles.

Maid costumes are still tame compared to what can be seen here.

You get people standing next to the fountains, on the sculptures, etc., around Ohotnii Rad all the time (basically the centre of Moscow). Whoever snapped that pic had great timing. At the fountain next to the circus at Metro Station University, all we had were some girls in short shirts, an older and somewhat belly'd gent, and a young lad.

Oh, and one partially not-sober diver. Diving into water four feet deep.