Short answers, pulled from the story.
Archaeological evidence from the 2nd century AD suggests Maslenitsa may be the oldest surviving Slavic holiday. This ancient celebration predates Christianity in Eastern Europe by many centuries.
People burn her effigy in a large bonfire to mark the end of festivities and bury her ashes in the snow to fertilize the crops for spring. Left-over pancakes are also thrown into the fire along with the straw figure during this ritual.
Orthodox Lent starts on a Monday unlike Western Christian traditions where it begins on Ash Wednesday. Easter determines when this specific week occurs since it moves annually based on lunar cycles which causes the date difference.
State atheism policies made public celebrations even less common following the Russian Revolution in 1917. The holiday survived primarily through family gatherings rather than large community festivals during these decades under official state campaigns.
Moscow has staged a yearly Maslenitsa festival next to Red Square since 2002. That celebration and other similar events attracted around 300,000 visitors in 2011 alone.