For the women - Nice slacks with a blouse and blazer or skirt, not too short, conservative business casual. Comfortable shoes, flats.
For the men- same, business casual- khakis and shirt with a collar, tucked in with a belt. Have seen some young men come in suits as well.
You want to be comfortable. It's a long day and you will likely do a lot of walking along with lunch with the regiment, a visit to the NEX etc.
Enjoy the day and connect with your future classmates.