According to Professor Quirrell, this is in order to protect Harry Potter (PS17), although of course the Gryffindors assume that he’s there to cheat in favour of Slytherin House. Harry catches the Snitch just a few minutes into the match to seal a win (PS13).
The match starts later in the afternoon, around 4 pm, which is a change from the normal 11 am start time. If Harry hadn't managed to catch the Snitch in record time, they would have been playing the dark. In fact, Harry leaves the stadium right at sunset, around 5:30 pm.
The start time for the match is derived from this passage (emphases mine):
Harry left the locker room alone some time later, to take his Nimbus Two Thousand back to the broomshed. He couldn't ever remember feeling happier. He'd really done something to be proud of now -- no one could say he was just a famous name any more. The evening air had never smelled so sweet. He walked over the damp grass, reliving the last hour in his head, which was a happy blur: Gryffindors running to lift him onto their shoulders; Ron and Hermione in the distance, jumping up and down, Ron cheering through a heavy nosebleed.
Harry had reached the shed. He leaned against the wooden door and looked up at Hogwarts, with its windows glowing red in the setting sun. Gryffindor in the lead. He'd done it, he'd shown Snape....
And speaking of Snape...
A hooded figure came swiftly down the front steps of the castle. Clearly not wanting to be seen, it walked as fast as possible toward the forbidden forest. Harry's victory faded from his mind as he watched. He recognized the figure's prowling walk. Snape, sneaking into the forest while everyone else was at dinner -- what was going on?
Sunset in mid-February in Scotland is around 5:30, so Harry would have been thinking over the "past hour" around that time. That puts the end of the match around 4:30, and the fast catch of the Snitch then puts the start of the match at 4 pm. This is the only time a Hogwarts Quidditch match starts at any time other than 11 am.
In the books, this match usually takes place towards the end of February. The fourth Saturday in February is the 22nd.
Granted this is the first book and things such as Quidditch start times were not established as Rowling was writing. It certainly seems like Rowling chose this time simply to have it be dark when Harry followed Snape into the forest.
However, even given that, it seems unlikely that they would have started a Quidditch match just an hour and a half before sunset. If Harry hadn't caught the Snitch in record time, they would have had to call the game because of darkness.
This is the only time in canon that we see Snape riding a broom. He was riding a broom in the Battle of the Seven Potters, but we don't actually "see" that incident, only hear it reported.
Lexicon timeline of Quidditch
Lexicon list of School Quidditch Teams
From the Web
WizardingWorld.com (Pottermore) features:
- A Blaggers guide to Quidditch
- Ranked: the best and worst Hogwarts Quidditch players
- Which Quidditch position would be right for you?
- First year Quidditch quiz
Harry Potter Wiki: Quidditch