He wrote to his parents in the second week of December.
The letter was not long. He explained that he intended to stay at Hogwarts for Christmas — the Tournament, the Yule Ball, the specific quality of a year that required being present in the castle rather than away from it. He told them he would be home for the last few days of December. He told his mother that the broth he had made the night before the first task had been wonderful and that he would make it for the family when he returns, which was the kind of thing that communicated more than it said.
His mother wrote back in the specific register she used when she had feelings about something and had decided to be practical about them. She said she understood. She said the first week of January was already on the calendar. She said his father had been telling everyone at the Ministry about the Tournament and was apparently the foremost authority on it among his colleagues, which she described with the specific fond exasperation of someone who found this equally embarrassing and endearing.
His father's letter arrived separately, as it usually did, in the slightly formal handwriting of a man who could not entirely turn off the Ministry correspondence style. It said:
Your mother is putting together a recipe file from all the things you've cooked for us this year. She started it in October. She hasn't told you because she wants to give it to you in January. I am telling you now so you can appear appropriately surprised. — Dad.
He read this twice and put it in the inner section of his trunk alongside the other letters he intended to keep.
Hermione's parents had been similarly understanding — he knew this because she had told him on the Tuesday with the settled quality she had when her logistics were in order. Her mother had sent a card that arrived alongside the confirmation letter, addressed to both of them, which Hermione had put on the dormitory noticeboard with the slight self-consciousness of someone doing something deliberate and having decided the deliberateness was worth it.
Harry was staying. Obviously.
Sirius had written in the second week with the quality he had in letters when he was managing his own anticipation of something. He would arrive Christmas Eve. Officially — McGonagall had been informed, the Ministry documentation completed with the thoroughness of a man who had spent a year rebuilding his legal standing and was not going to let a Christmas visit become a complication. He would stay through Boxing Day.
Harry had read this letter at breakfast and said nothing for approximately thirty seconds and then said, 'He's coming,' in the tone of someone who had been told something good and was still working out where to put it.
'Yes,' Ron said.
Harry folded the letter carefully and put it in his pocket and went back to his eggs.