A free period is a second period that a girl will get in one month. A girl is supposed to have one period each month, if she has a second one in the same month, then it is a free period.
Considering the fact that an egg is, for all intensive purposes, a chicken's period, and vodka has for many generations been the drink of choice for Russians. It is only appropriate that a Russian Period be a raw egg floated atop a shot of vodka.
Did you see the Russian Period Matt just drank? I hope he doesn't throw up!