Duvet day: A day off from work, taken simply because you don't feel like going into the office. Such a day might be spent in bed, snoozing under a duvet (doo-VAY or DOO-vay).
According to Word Spy, the concept of "perks for the pooped" was invented in 1997 by British public-relations firm August.One. The public-relations consultancy Text 100¹, which is based in New York City and has offices in several countries, adopted the practice--and the term--shortly thereafter. Text 100's "careers" page includes this explanation:
In addition, we offer two personal ”duvet’ days” a year in addition to generous vacation leave. We believe people are at their best when they have the opportunity to manage their lives beyond the office.
"Duvet" and "perk" (an abbreviation of "perquisite") are imports into American English from British English; their American counterparts are "comforter" and "fringe benefit," respectively, but the UK equivalents have been very successfully transplanted to North America.²
American equivalents of "duvet day" include "mental health day" and "calling in well," which are descriptive but lack the rhyme, alliteration, and indolent associations of the British term.
¹ Text 100 is on the passion bandwagon, according to the "careers" page: "We look for people who combine a passion for technology with an ability to work collaboratively." It's unclear what the company would do with a candidate who combined an ability for technology with a passion for working collaboratively.
² Can any reader confirm when that adoption occurred? Both words were unfamiliar to me in the 1970s, when I first heard them from speakers of British English.