The first allowable date for an official visit is the first day of classes in the senior year of high school.
In my opinion, it is better to wait to make an unofficial visit until the player has done something to create strong interest. For example, scoring 9 or 10 at a PG showcase, playing (and contributing) on a top rated travel team, or attended and stood out at a camp at the college. Actually in that latter case, if the school is interested, they'll typically let you know at the camp.
If a school is interested, they will do a pretty good job of showing you the school and program. But it's a big committment of time, and they can't really do that for everybody who hopes to play at the school. They'll also try to get you there in the fall, when the baseball team is practicing, and for bigger schools, when there is a football or perhaps basketball game. The visit will typically have an agenda, the visiting player may stay in a player's room, etc. This is hard to arrange before Sept 1 of the junior year, since the coaches can't send emails or letters until then. And they can't call or text.
On a visit, the player can't do anything to show his worth to the coaches. So from a coach's perspective, the point of a visit is to sell you on the school. They only want to do that for the players they are interested in. If there is a particular chool you are interested in, attend one of their camps. The only way to get interest is to display talent.
