Масленица (Maslenitsa) is often called “pancake week” (blini) or “cheesefare week” as it falls annually on the week just before Great Lent.
Great Lent is the 40 days of fasting before Easter, with no meat, fish, eggs, dairy, etc, consumed until Orthodox Easter.
Maslenitsa is also associated with the beginning of spring. It is celebrated in most of Eastern Europe and parts of Asia.
Interesting reading here: http://russiapedia.rt.com/of-russian-origin/maslenitsa/