Budapest Wine Festival
I am the author of the Frommer's travel guide "Budapest & the Best of Hungary", 7th edition. The wine festival happens the first weekend of September. Every year, one additional country is invited to display and sell their wines. The last couple of years, it was Spain and South Africa.
Before the festival, there is a wonderful parade with the first bread of the season, many in old costumes, dancing to music along the way. Much of the fun is watching and then following the parade. The bridge that they usually cross is under reconstruction, so the route will be different this year. You should check with TourInform when you get here as I could not get a clear answer now in July.
You will pay an entrance fee for the festival, but that includes a wine glass and some starter tickets to get your first tastes. Additional wine tickets are sold throughout the festival and wines prices are variable, but always sold by the dl (deciliter).
If you have question for me, please write to me directly at drryanjamesATgmail.com otherwise I may never see them.
I looked around for you, but I couldn't find much information in English. Search results in Hungarian were better but not much.
English Description from Frommers:
Event Calendar in Hungarian:
If you'll be there between September 8-14, you should be able to catch most of it.
I haven't attended so I'm not really sure what each event really entails. I understand that the coolest stuff happens at the Castle (var).