If you have fun solving tricky questions with a scientific background during pre-Christmas period, you should click on the WWU Online Advent Calendar. There will be a maths question as well!
University of Münster's Advent Calender [de]
For children, teenagers and school classes, the website "Mathe im Advent" is recommended. Every day you can find new puzzles for different ages.
"Mathe im Advent": Calendar for children and schools
The "MATH+ advent calendar" offers insights into recent application oriented mathematical research and into the everyday professional life of mathematicians. The 24 challenging puzzles are aimed at sharp high school students, university students, teachers, and everyone interested.
With the Advent calendar "Krypto im Advent", students in grades 3 to 9 can discover the world of encryption.