They were in seventh year when it happened. He sat at their table for the start-of-term feast, for breakfast, lunch, and dinner the next day. Draco left him alone, thinking it was a dare or something. He began to doubt his assumption after a week of seeing Potter angrily stomping away from Gryffindor Table. Draco had watched as he punched the Weasel (who walked away with a broken nose), and made the Mudblood cry (she tried to silencio him, but he deflected it and pointed his wand at her).
Harry Potter had sat at Slytherin table for a month before Draco apporached him.
(Voldemort hasn't been defeated yet)