A bit of cultural background rather than tricky words today. It's early December, the Advent period has begun, and Christmas is approaching. This name is a link back to the time before the year 1600 when England, Wales and Scotland were Catholic countries. Each of the saints had a specific day in the year for mass (omša) to be said in his or her name, and Christ's mass can be said from dusk (sunset) on December 24th onwards.

Other "-mases" which English-speaking people might know about are Candlemas on February 2nd (Hromnice), celebrating the purification of the Virgin Mary 40 days after Jesus' birth; Lammas ("loaf-mass") on August 1st, celebrating the first harvest crops (loaf is "bochník chleba"); Michaelmas on September 29th (Michaelmas daisies are wild asters) and Martinmas on November 11th. English-speaking people don't normally use these names now, just like they don't celebrate name-days.