Well, MLB stands for Major League Baseball. So I'm guessing a Fanfest would be some big party/convention for fans. Don't know how this will affect the parks though. Maybe they will be so in to base ball that they all stay away. Or... the one person in the family who likes baseball talks the rest of the family in to going by saying they can go to DLR.

We went last June during a giant convention of teenagers and I never noticed the difference in the parks.
We are thinking of going end of July or maybe first week of August.