I would go in the morning for two reasons.
1. It's an all day ticket so you may spend as much time there as you wish.
2. The queues for entrance are usually huge this time of the year so the earlier you start the better.
if you get there by 10.30 monday-saturday will be able to see the world famous Changing the Guard otherwise known as the Guard mount, at approximatly 10.40ish the new guards march from nearby Victoria barracks after the ceremony in the castle the old guard march back to the barracks.