According to OPM, “it is a legal public holiday for pay and leave purposes only for Federal employees who work in the ‘Inauguration Day area’.”
That area, says OPM is “defined in law as the District of Columbia, Montgomery or Prince George’s Counties in Maryland, Arlington or Fairfax Counties in Virginia, or the cities of Alexandria or Falls Church in Virginia.” Fairfax City is considered to be part of the county in this case.
So if you work in the zone, you have a four day weekend.
If you’re travelling outside the area, you’re expected to work the day.
On the other hand, if you’re travelling to the Inauguration Day area, you get the day off.
Thrown into the mix is the question of non-career (political) appointees who are resigning on the 20th. Those working in the Inauguration Day area get the holiday, those working outside do not. Rule of thumb: resignations take effect at noon for most of the non-career SESers, so pay will only cover until then.