"Mummy will never forget you!"
--Rubeus Hagrid (PS14)
Using the invisibility cloak, Harry and Hermione sneak Norbert up to the top of the astronomy tower, where Charlie’s friends successfully attach Norbert’s crate to their brooms and fly away with the dragon without being caught. Unfortunately, Harry and Hermione forget the invisibility cloak at the top of the tower are caught by Filch (PS14).
Derived from the estimated date of Norbert's hatching. We know from Charlie's letter that the Charlie's friends showed up at the astronomy tower at about midnight on a Saturday (PS14). It is 1:oo in the morning when the McGonagall gives them detention (PS15).