In the past two seasons the Nats' lowest attendances have come in April:
2012 - Wednesday 18th April vs Astros (14,520) - weather 53 and overcast
2011 - Tuesday 12th April vs Phillies (13,413) - weather 55 and overcast
[weather cites from ESPN.com match reports]
I think I'll be dissapointed if we have those kinds of numbers again after the season the Nats just had. School, weather, whatever. We'll see if the theory holds true that attendance lags one year behind on-field performance.