Comparative dates on which the Nationals notched their fifth win, each season (sorry I didn't post this sooner but I have no internet access at home):
2014 - 8th April (7th match of season)
2013 - 9th April (7th match)
2012 - 12th April (7th match)
2011 - 12th April (10th match)
2005 - 13th April (9th match)
2010 - 16th April (10th match)
2015 - 17th April (11th match)
2006 - 18th April (14th match)
2007 - 18th April (15th match)
2008 - 18th April (17th match)
2009 - 29th April (20th match)
Comparative dates on which the Nationals notched their sixth win, each season:
2014 - 9th April (8th match of season)
2013 - 10th April (8th match)
2012 - 13th April (8th match)
2005 - 14th April (10th match)
2011 - 15th April (13th match)
2010 - 17th April (11th match)
2015 - 19th April (13th match)
2006 - 20th April (16th match)
2007 - 20th April (17th match)
2008 - 22nd April (21st match)
2009 - 2nd May (23rd match)