substitute holiday; compensatory day off
Abbreviation of 振替休日. Refers to a day off given in lieu of a national holiday that falls on a Sunday, or more generally a compensatory holiday for working on a holiday.
See also: 振替休日 (ふりかえきゅうじつ)
This Monday is a substitute holiday, so the office is closed.
I worked on a national holiday, so I'll take a compensatory day off on Friday.