Primary and Secondary Maths advent calendars from NRICH.
The primary advent calendar has 24 activities, the tasks focusing on encouraging mathematical habits of mind: being curious, being thoughtful, being collaborative and being determined.
The secondary advent calendar includes both short and longer tasks to encourage mathematical creativity.
Primary Advent Calendar: http://nrich.maths.org/12902
Secondary Advent Calendar: http://nrich.maths.org/12956