This question has been asked and answered a lot on this site, so I figured I could do a quick search and could find it, but I couldn't.
There are a lot of details, but the bottom line is this:
Coaches can send a questionnaire or camp invite at pretty much any time. 2B started getting questionnaires between his freshman and sophomore year. Ours were not a result of any emails that were sent out - they came after his first summer doing the 16U tour - East Cobb, USA Baseball, etc. (he was 14)
Usually there was a cover letter saying they couldn't "recruit" him at that time due to NCAA rules, so please fill out the questionnaire for their files. They may have referred to him as a "potential recruit" or something like that.
Then on September 1 of the junior year, a player can receive direct email from the coach that is much more specific about their interest. On July 1 between the junior and senior year is when the coach can phone a player directly.
Players can call coaches at any time.
To find out the baseball-specific recruiting calendar, just do a search on "recruiting calendar" and there is tons of information here.