Tweede Kerstdag is the Second Christmas Day, like Boxing Day in the UK. It is an official holiday in the Netherlands.
The Second Christmas Day was introduced to facilitate going to church multiple times, and to have some time off for contemplation and relaxation. Nowadays, the Second Christmas Day is often used to visit in-laws, go to a concert, or do some shopping in furniture malls.
Prettige Kerstdagen! (Happy Christmas Days)