Albus never celebrated Christmas. Especially after moving to Godric’s Hollow. He wanted to believe those traditions were alien to him and diligently ignored certain thoughts.
The thoughts he suddenly had when he looked at Gellert. It might have been the fifth or the sixth day of their combined lessons. They were practising magic far more complicated than the magic they were supposed to study in their year. They were absorbed in reading about blood magic and were trying practising legilimency. Albus was looking at Hellert foolishly thinking about Christmas mistletoe at the height of summer.
Later they began trying joint spells, testing their bond and the limits of the possible. “Together we can do anything”, Gellert said. “Even the mistletoe thing”. Albus laughed.