Today is April 15, and normally today would be tax day, only when the 15th falls on a Saturday or Sunday the date due is advanced to the next business day.

So, you would think that tax day would then be April 16. Well, on April 16, 1862, nine months before he would issue the Emancipation Proclamation, President Lincoln issued a special emancipation of all slaves in the District of Columbia, basically by giving compensation to their owners.

As such, April 16 was made a holiday in the District of Columbia, so whenever tax day moves to April 16, since that is a federal holiday in that city, the tax date (for the entire country) is moved forward to the next business day, so that's why tax day falls on April 17 this year.