50 days
Because its a calendar counting down the days of advent.
The origins of the Advent calendar come from German Lutherans who, at least as early as the beginning of the 19th century, would count down the 24 days of Advent physically...
The communists needed a day to meet and celebrate the Haymarket martyrs, and Joseph Dietzgen decided to pick the first of May.
Colombus day will be October 11th, 2010.