Washington DC celebrates Emancipation Day on the weekday nearest April 16th. Wikipedia says:
As the federal government observes the holiday, it causes the federal and all state tax deadlines to be moved to the 18th if Emancipation Day falls on the weekend and to the 17th if Emancipation Day falls on a Monday
under a federal statute enacted decades ago, holidays observed in the District of Columbia have an impact nationwide
which maybe is true, but since the Post Offices in DC are open, it is hard to see the impact. There is no link to the relevant federal statue.
Why does Tax Day sometimes depend on Emancipation Day>