I had to go look it up, since I was so confused. I never knew this!
From the timeanddate website:
"In 1913 Tax Day, or the filing deadline, was fixed on March 1. However, it was moved to March 15 in 1918 and April 15 in 1955, where it has remained since then. If April 15 falls on a Saturday, Sunday or a civil holiday, such as Patriot's Day, the deadline is extended to the next working day. An extension due to a holiday may only affect certain states. In 2007, the residents of some states were granted an extension due to the disruption to public life in many areas caused by a huge Nor'easter storm. In some years in Washington DC, Emancipation Day may be the reason to extend the deadline for filing an income tax return (Tax Day). In 2007, the observance Emancipation Day in Washington DC had the effect of nationally extending the 2006 income tax filing deadline from April 16 to April 17. This 2007 date change was not discovered until after many forms went to print.
Weekday Date Year Name Holiday Type Where It is Observed
Thu Apr 15 2010 Tax Day Observance
Mon Apr 18 2011 Tax Day Observance
Tue Apr 17 2012 Tax Day Observance
Mon Apr 15 2013 Tax Day Observance
Tue Apr 15 2014 Tax Day Observance
Wed Apr 15 2015 Tax Day Observance
Mon Apr 18 2016 Tax Day Observance
Tue Apr 18 2017 Tax Day Observance
Tue Apr 17 2018 Tax Day Observance
Mon Apr 15 2019 Tax Day Observance
Wed Apr 15 2020 Tax Day Observance"