As Nev says, you've missed the early summer events. What you could do, instead of going to an individual winery, would be to visit one of the caveaux where you get the chance to taste wines from different winegrower in the area. Those in Riex, Rivay, Epesses, Chexbres, St Saphorin etc are all open in the early evening.
More info here:
http://www.lavaux.com/navigation.asp?page=caveaux.asp
Hope you have a great weekend....looks like the weather will be on your side!