its my understanding that only feasts of the Lord will fully override a Wednesday or Friday fast.
Not all Dominical Feasts: the Feast of the Transfiguration always falls in the Dormition Fast, and traditionally fish and oil are eaten, but not meat or cheese. Palm Sunday allows fish and oil (according to some, only oil), but again, no meat or cheese. The Exaltation, of course, is always a fast day. Some consider the Meeting to be a Dominical Feast, and again, fish only if it falls on a fast day. Last year it actually fell on Clean Monday for old calendarists, and we used oil, but not fish.
Although it's not usually considered among the Great Feasts, Mid-Pentecost, which is always a Wednesday, allows fish; in our church we actually have a tradition of using cheese and eggs as well, but not meat. Likewise with the apodosis of Pascha.
The only Dominical Feasts I know of that actually call for a complete relaxation on Wed and Fri are Christmas and Theophany.
As Cymbyz noted, Great Feasts of the Theotokos also allow fish: Nativity, Entry, Meeting, Annunciation (unless during Holy Week), Dormition. Likewise SS Peter and Paul, and I think most would say the Nativity of the Forerunner as well. Also the Synaxis of the Forerunner (day after Theophany) allows fish.
I've seen Russian rules that are much more relaxed, with fish allowed on several other feast days, but this is not normal for the Greek church.