flu day
n. A day during which a school or school district is closed to prevent or reduce the spread of the flu.
Closing schools or shutting the subways might be even more effective, because children are much more efficient than adults at spreading flu, and subways are enclosed spaces where sneezes linger in the air — but doing that would be harder to pull off, Dr. Weisfuse said. "People talk about 'flu days' like snow days," he said, "and if it was just days or a week, that would be simple. But if it's weeks or months, that becomes another matter."
—Donald G. McNeil Jr., “Greetings Kill: Primer for a Pandemic,” The New York Times, February 12, 2006
First it was snow days. Now, many Northern Kentucky schools are facing "flu" days.

So many students in Fort Thomas are sick with flu-like symptoms that the school district has closed its five schools for today and Friday.
—“Some N. Kentucky schools closing because of illnesses,” The Cincinnati Enquirer, January 30, 2003
1989 (earliest)
Oklahoma public schools that canceled classes because of influenza outbreaks must make up those missed days this semester, the state Board of Education said Thursday. …

The flu days must be made up, the board decided.
—Jim Killackey, “State Schools Told to Make Up Canceled Classes This Semester,” The Daily Oklahoman, March 24, 1989
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