Fastelavn

Fastelavn is the carnival celebrated in Scandinavia on the Sunday before the beginning of Lent.

http://en.wikipedia....ival_in_Denmark

On the following day, Fastelavnsmandag, Danish children go to school dressed up in costume. This year the return to school after winter break coincides with Fastelavnsmandag, so Prince Christian and Princess Isabella would have participated in this traditional festival too.