"Dobby volunteered to return Harry Potter's owl!"
Harry, having fallen asleep in the common room, wakes up from his recurring dream to find Dobby before him. The elf is wearing a tower of knitted hats with a cured Hedwig on top of them. Dobby tells Harry that Winky is still drinking and struggling emotionally, and asks Harry about the dream that been causing him to mutter in his sleep. Harry asks Dobby if he knows of a suitable place for D.A. meetings, and Dobby tells Harry about the Room of Requirement (OP18).
This occurs very early in the morning, when it is still dark out and most people are asleep (OP18).