If you have the test kit in your hands before the water starts filling, you're going to be in great shape.
You know you'll need stabilizer. So you can start shopping for that. You know you'll need bleach, so you can hunt for bargains there, too. When you're out and about, look to see who sells acid and washing soda and borax. You don't which, if any, you'll need until you can test the pool water, but it's good to know where to go so you're not frantically searching on opening day.
Look for pool toys on sale as well.