Estringokl was a human colony and the location of the festival Planetfall. The Doctor visited Estringokl when it was frozen over, and promised a dying robot that he would continue its work of decorating the Christmas tree each year.
The Third Doctor, his memories of Estringokl having been taken away by the Time Lords, took Jo Grant to Estringokl for the festival, but discovered it frozen. He remembered his promise, and he and Jo continued to decorate the tree for ten Christmases until the colonists awoke from cryonic suspension. (PROSE: The Thousand Years of Christmas)