If you haven’t used a sick day yet this year, you just might in a few weeks—if you’re like a good deal of American workers, anyway.

Aug. 24 is the nation’s “sickest day of the year,” according to new research by leave management platform Flamingo Leave Tracker. The company analyzed five years’ worth of data, looking for the most common dates that workers call in sick.

What they found: “The sunny days of late summer showed a more significant drop in attendance than the frosty winter months typically associated with colds and the flu.”

Make of the finding what you will. But the day approaches as the U.S. endures its fourth COVID summer—and as levels of the virus in wastewater, and associated hospitalizations, rise throughout the U.S.

COVID on the rise

Nationally, COVID wastewater levels are similar to that of every pandemic summer except for 2022, when the world saw a spike due to Omicron variant BA.5. That’s according to BioBot Analytics, which monitors COVID wastewater levels for the federal government.

The data makes the case that COVID is not merely a winter respiratory virus like flu and RSV, but a summer one, as well.

“By and large, outbreaks of COVID-19 are settling into a sort of seasonality, with predictable spikes after national holidays like July Fourth,” Tom Cotter—executive director of nonprofit Healthcare Ready, told Fortune.

The juries of some experts, however, are still out on summer seasonality. Since antibody immunity—from infection or vaccination—only lasts three to six months, it’s hard to definitely tell if the virus truly favors the summer and winter seasons, or if such spikes are simply a result of fading immunity.

When and why employees call out sick

The second sickest day of the year, Feb. 13, comes during the traditional winter respiratory illness season. But it’s also suspiciously aligned with the Super Bowl and Valentine’s Day. We’ll leave you to draw your own conclusions on the root cause.

The third through fifth sickest days of the year, according to Flamingo’s report: Oct. 25, Dec. 13, and April 18.