The Christmas stamps are in

The December stamp is a stamp with a special rate for sending Christmas cards and New Year's greetings. The stamp, therefore, has limited validity.

Decorating a tree, sending cards.
On the December stamps, the richly decorated Christmas tree plays the leading role as a beautiful symbol for this special month. When designing the stamps, Loesje Donner-Raedts van Vondels was inspired by beautiful decorations and unforgettable moments. Such as the family dinner in December, the decoration of the Christmas tree, being together with loved ones, and the long-awaited moment when the new year arrives. And of course, we let others know that we are thinking of them with a card. Because every ticket is a nice gesture.
Source: text on the back of December stamps.