Taste of Wheaton - way cheaper than Bethesda's, with parking closer in - is Sunday May 16 near the Metro station.
The Greek Fest at the church on Bradley Blvd nr 7 Locks Rd in Bethesda is the 1st weekend of May.
The Greek Fest at the church at 36th & Mass (in the shadow of the National Cathedral) is Mother's Day weekend.
The Greek Fest at the church on Cipriano Rd in Greenbelt is June 11-13.
There are also Greek churches on upper 16th St DC, & at 7 Corners in VA, also one in Annapolis & downtown Baltimore, but I haven't found the dates of their spring food festivals yet. The 2 more local churches purposely plan their festivals on different weekends than the above churches. Nobody wants to take away the other's customers...
Great, cheap food & live entertainment or music at all places!