Swimmers and spectators at Coney Island’s 121st annual polar bear plunge will find the weather a touch above frigid – in the low 40s – after an unseasonably warm week begins to moderate over the weekend.

This isn’t to call the Coney Island polar bear an endangered species, as “The Daily Show” once quipped after an unseasonably warm winter.

The New Year’s Day event expects some 4,000 swimmers and 20,000 onlookers this time, organizer Dennis Thomas said.

“Jumping in the cold ocean is a fantastic way to clear your head and bring in a new year with optimism,” Thomas said, adding that his club partakes every weekend from November through April – Jan. 1 is simply when they invite the public to join in.

“It’s definitely a break from the past and a break from your day-to-day routine,” Thomas said.

The Coney Island Polar Bear Club was founded in 1903 by Bernarr “Body Love” Macfadden, a weightlifter, pulp fiction publisher and alternative-health proponent who changed his name from “Bernard” because he believed Bernarr sounded like the roar of a lion.

The club has canceled its New Year’s plunge only once since then, in 2007, when temperatures in the city broke 70 degrees.

The forecast for Monday’s plunge puts the outdoor temperature in the low 40s at 11 a.m., when the collective dip begins. And water temperatures in New York Bay are hovering around 45 degrees – colder than the cold plunge pools at most of the city’s spas and schvitzes.

Participation is free, but aspiring plungers must register at the Polar Bear Club’s website.

Same-day registration is available starting at 10 a.m. at tables outside the Coney Island-Stillwell Avenue station. Swimming begins at 11 a.m. and goes until 2 p.m.

The organizers accept donations, which they say go toward local community groups including the Alliance of Coney Island, the New York Aquarium, Coney Island History Project and others.

“We leave Times Square to the tourists on New Year’s Eve,” Thomas said. “But New Yorkers come to Coney Island.”