There hasn't been a soft opening (castmember preveiw) the day before the start of the F&G festival in past years. Even for Food and Wine the castmember preview, which is open to anyone at Epcot, is the day before so I doubt there would ever be a preview the week end before.
As Tess mentioned there is a Passholder's Preview event, but that is a paid event and something very different from the castmember preview.