Harry and the others had not told Draco about their plan to send the baby dragon away. Although Draco had not provoked them during this time, he was still nowhere near earning their trust.
The agreed time was Sunday night. Naturally, the sooner they dealt with something like this, the better.
When evening came, Harry and Ron's earlier excitement began to give way to nerves. Harry could barely taste his dinner, but since they had something to do that night, Hermione and Neville persuaded him to force a little more food into his stomach.
Ten o'clock was curfew at Hogwarts. At that hour, unless students were coming out of the Library, being caught by a professor or member of staff would cost them house points.
There were exceptions, of course. If Snape caught them, they would most likely lose points even if they were coming out of the Library. So most young witches and wizards returned to their Common Rooms before eleven. There was no time limit there; if they wanted to stay up all night doing homework or anything else, they could.
After nine, though, few students wandered around the Castle. After all, there were no electric lights in the Castle. Although torches on the walls provided illumination, they were actually sustained by magic, and their flames shrank very small after nine.
This left the corridors pitch-black after nine. Even among Gryffindor's bold little lions, not everyone was willing to go wandering around without a particular reason.
Of course, as Charlie Weasley had said before, a Gryffindor who had never roamed the Castle at night was no proper Gryffindor. Though they were not especially fond of it, many of the little lions had done it more than once.
Harry and Ron had hidden the Broomstick in the Astronomy Tower