Alone because he has no one travelling with him right now, the Doctor wanders through his TARDIS, room after room, looking in forgotten cupboards, finding mementos left behind by past regenerations and companions long gone.
That’s how he comes across the Santa hat; it’s a bit dusty, but a quick once-over with his sonic screwdriver fixes that.
He likes hats, they’re cool, so he puts in on, but it’s too big and falls straight down over his eyes. He smiles; Father Christmas was a big man all over, big hair needs a big hat.
1952: that was a good year.