Not sure of the exact time, but I'm pretty sure it is open whenever the park opens for the day.
In Nov. we went on an EE day to MK and after we rode all the Fantasyland rides we had wanted to get we had a hard time finding a place open to get a bite to eat. A CM directed us to the Main St. Bakery, so it must be open early. Oh, wow does it have some good stuff!!! Yum!!
Gi