Today's Nationals-Pirates game has been rained out, the Nationals just announced to the media. The team will have more information on a makeup date in the coming days.
The Pirates are back in town over Fourth of July weekend, in a scheduling quirk where those two teams play while everyone else in baseball has an interleague series, so my guess would be they'd just play a doubleheader at that point.
Unofficially, the Nationals should be able to just bump all their pitchers back a day and be fine, since there's an off-day after the Brewers series next Thursday. (And for my fellow cheeseheads in the audience, that means Auburndale, Wis., native Jordan Zimmermann would pitch in his home state for the first time; the rotation didn't line up two years ago, and he was hurt last year.)
I'll have more info on makeup possibilities and how this affects pitching matchups soon. Stay tuned.