"She was the one who started it ...I wouldn't've - she just sort of came at me - and next thing she's crying all over me - I didn't know what to do"
-- Harry Potter about Cho Chang (OP21)
After the last DA lesson before Christmas, Cho Chang remains behind in the Room of Requirement with Harry. They talk about the DA and about Cedric’s death, she starts to cry, then they kiss. Later, in the common room, Harry describes the experience as “wet.”
We know it's a Wednesday because Hermione says two days later that it's the last day of term, so with the calendar we've been using for the term, that would make this 18 December.