You'll see them both in the fall and in the spring. As a junior you have to pay your own way for everything except they can give you and your parents three tickets to a game (football in the fall, basketball over the winter, baseball in the spring). You usually get a group tour and a presentation from the coaching staff, maybe also from the academic support people.
It's a great way for them to show interest, for you to show you're interested, and to get a much closer look.
Also it's not unusual for offers to be extended or negotiated on junior day visits, if you get off to the side alone. NCAA rules let you talk to the coaches for as long as you are both willing, as long as you're standing on their soil.
But that doesn't mean you should feel pressured to accept anything on the spot, nor disappointed if you don't get an offer then and there. Patience pays.