There's a good reason for this. Children tend to feel most comfortable and in control when the same things happen at the same time each day. Start by offering meals on a regular timetable. There are art projects to do, field trips to take, and playgrounds to explore. Another thing to consider is how and when your child needs to nap.
Preschools usually schedule nap time after lunch.
If your little one can keep going until then or even all day like a wind-up toy, he's set. If he still needs a mid-morning snooze, it might not be time yet to go to school. You can work toward building his stamina by making sure he gets a good night's sleep. If you have some flexibility in your schedule, you might also want to start him off in a half-day program to ease him into the hustle and bustle of preschool life, and gradually increase the length of his school day as he gets more comfortable. Think carefully about what your goals are for sending your child to preschool.