Really? That's very strange, I thought it was a sellout.
What's even stranger is that this 34,789 figure is a Nats Park record for any federal holiday (that is, the date on which the holiday is observed and we feds get the day off) - we haven't even broken 35k, let alone sold out:
2008
Memorial Day - 28,552 vs Brewers
Fourth of July - away to Reds
Labour Day - 28,393 vs Phillies
2009
Memorial Day - away to Mets
Fourth of July (observed Friday 3rd) - 33,982 vs Braves
Labour Day - travel day
2010
Memorial Day - away to Astros
Fourth of July - 29,234 vs Mets
Labour Day - 20,224 vs Mets
2011
Memorial Day - 34,789 vs Phillies
Fourth of July - 32,937 vs Cubs
Labour Day - 25,518 vs Dodgers