The eighth years’ idea to watch the sunrise on Graduation day came from Harry’s idea to watch it on the day after the first anniversary of the Battle of Hogwarts.  He said he’d need to see the sun for himself to know that the day was truly over.  Anyone from the castle was welcome to join them, as such, the shore of the Black Lake was filled with students and professors that morning.  They all needed proof that the sun would rise again.