While the actual day is the 6 December, on the night of 5 December you will see children and adults alike dressed as St. Nicholas, an angel or a devil.  The tradition here in the Czech Republic is that St. Nicholas and his helpers are searching for the children to determine if they were naughty (in this case the devil will give them a piece of coal or if they were very, very naughty he will drag them away) or those who were nice (they will receive a bag of sweets).  Children tremble but never fear as the angel is there to protect all children from the devil!  The children are also required to recite a poem or sing for St. Nicholas.  This year it might be more contained but I am sure you will be able to enjoy some activities around the city.