Diagon Alley is usually open during the School Sorting because it allows everyone to RPG their school characters buying supplies, meeting up with friends, etc. It also allows newcomers to the school RPG to do the same and to interact with members as they establish their school character.

DA usually closes around the same time the sorting ends (sometimes, it stays open for another day or so). So don't panic when you can no longer see DA! If you were not done RPG your shopping trip, then pretend that you've purchased everything you need.

The Hogwarts Express usually begins several days after Diagon Alley closes (less than 7 days).