Washington ? ‘Tis the season for making whoopee.

The Christmas-New Year’s period produces a year-high spike in sexual activity and conceptions in the United States, according to biorhythm researchers and makers of sex-related products.

They attribute the increase to holiday leisure and New Year’s resolutions to have children. New Year’s irresolution fueled by alcohol and partying is another contributing factor.

“Right before New Year’s Eve is our highest sales peak,” said David Johnson, group product manager for Trojan brand condoms, the leading U.S. seller.

As expected, the holiday urge surge also expresses itself as a peak in U.S. births in September, according to David Lam of the University of Michigan’s Population Studies Center in Ann Arbor.

Holiday intimacies aren’t just an American rite, according to Gabriele Doblhammer of the Max Planck Institute for Demographic Research in Rostock, Germany.

Heavy Christmas-New Year’s sex “is characteristic of all Christian cultures in which it has been evaluated,” she and co-researcher Joseph Lee Rogers found.

A quartet of British public health researchers went so far as to liken the Christmas-New Year’s period to a “festival on fertility” in a 1999 article in the Journal of the Royal Society of Medicine.

Such festivals, they wrote, are “associated with increased opportunities for socializing and a generally more hedonistic approach to life.”

Recognizing the risks entailed, the British Health Education Authority once ran a condom ad before New Year’s with the tag line: “Just in case old acquaintances aren’t quite forgot.”

What about the notion that spring is the season of love?

Among partners chronically pressed for time, intimacy flourishes in the rare leisure of three-day weekends, analysts said.

Accordingly, the long July Fourth, Memorial Day and Labor Day weekends also produce spikes in condom sales, Trojan’s Johnson said.

So does the run-up to Valentine’s Day, he added.

“And before Mother’s Day, there’s a small peak.”