Reflecting on your question based upon our recent experience. (Son is an '05, that signed in June with Labette CC in Parsons, KS)
We attended a D1 fall showcase in October. A few of the area JUCOs were present, but most were still playing their own "fall schedule". We attended an invitation only showcase at a specific JUCO over the Christmas break. That one did not work out for us, but there were about 30 kids there, and certainly some of them signed at the earliest chance in January.
Kansas is a LONG way from Montana, so we were not able to get back there until just after my '05s graduation in early June. At that point most rosters seemed to be mostly filled. There were two schools having open tryouts in early June, and we made arrangements for a private workout at two other schools. Son's signing came about as a result of the private workout at one of the schools.
So based upon our experience, the bulk of the recruiting happens after fall ball and into the early part of the season. By the end of the Juco season, most schools were really only looking for more pitching.
My advice if you know that JUCO is the route that you will be going, be proactive early. Make contacts with the schools that are high on your list and see if they have any fall or winter showcases that the Juco itself is putting on. Good luck in your endeavor!