You get an extra weekend to file your tax returns in 2016 thanks to an uncommon mix of holidays and weekends. Ordinarily an unextended Form 1040 Individual Income Tax Return is due on April 15th of any given year. However, when the 15th falls on a weekend or legal holiday, the due date of the tax return is the day after the weekend or legal holiday.
April 15, 2016 is a Friday. Without more, your income tax return would be due that day. Yet, the District of Columbia observes Emancipation Day on April 16. Where Emancipation Day falls on a Saturday, it is observed the day before. Where Emancipation Day falls on a Sunday, it is observed on the day after. Because Emancipation Day 2016 falls on Saturday, April 16, it will be observed in Washington D.C. of Friday, April 15. This makes the 15th a legal holiday. Altogether, this pushes the Form 1040 due date to Monday April 18th.
If, however, you live in Massachusetts or Maine, you will have until April 19th due to the statewide observance of Patriot's Day on Monday April 18, 2016.
For greater detail see IRS Revenue Ruling 2015-13.