It is the feast day of St. Wenceslas, the patron saint of Bohemia, and commemorates his death in 935. Also known as ‘Czech Statehood day’, it is a national holiday in the Czech Republic and has been a public holiday since 2000.

Wenceslas, a Christian in a pagan environment, is understood to have reigned with courage, dignity and piety.

As a footnote, and a warning against fratricide, it was Boleslav who founded the independent Czech state, yet his martyred brother has the Day of Czech Statehood celebrated on his feast day.

An article about history here.