Last Sunday, you may recall, the air temperature dipped to the low 20s, the region braced for its first snowstorm of the season and ice crystallized along the lake shore — in other words, the perfect day for a post-run plunge.

I’m not sure exactly when or why friends and I started this tradition — nutty, even by my screwball standards — but for several years we’ve taken to diving in at least once every month after a group trot.

Our secular baptism of sorts is an offshoot of the annual New Year’s Day run from Mystic that culminates in a lemming-like, group leap into Fishers Island Sound. This popular, celebratory exercise in mass hysteria that first kicked off more than a half-century ago now attracts hundreds of boisterous enthusiasts.

To my knowledge, only a handful of diehards quietly keeps the fun going all year at various locations, including Bluff Point in Groton, the Mystic River and Long Pond in Ledyard.

Of course, swimming outdoors is no big deal from April through October; the other months test the mettle.

Even though Sunday’s conditions were less than ideal, it was the first day of December, and therefore unlikely to warm up if we waited until later in the month. Carpe diem.

My son, Tom, neighbor Bob Graham and I began our run at 8 a.m. — Bob and I loped along for five miles while Tom tacked on a couple extra for good measure. We then rendezvoused for the moment of truth, stripped down to running shorts, stared apprehensively at shriveled lily pads and wind-whipped reeds, gritted our teeth, and prepared to leap into the slate-gray water.

At this point in any venture sure to bring discomfort, there’s often a pause for reflection, when you ask yourself, “Do I really want to do this? What am I trying to prove? Honestly, will anyone know, much less care, if I decide to bail?”

The key is to act before any such hesitancy allows common sense to intervene.

And so we edged over slippery rocks, waded up to our knees and leaned forward. We thus crossed the Rubicon.

“Yaaah!” I cried, held my breath and dove.

My brain may have known what was coming, but there was no way the rest of the body could prepare for the shock — a paralyzing jolt that felt as if Jermaine Eluemunor, the Patriots’ 6-foot-4, 335-pound offensive guard, had clobbered my forehead with a 2-by-4.

I may have stubbed my toe on a submerged log — it was hard to tell because I had lost all feeling in my feet — but managed to thrash a few strokes before my arms turned to cement. Seconds later, I stumbled, gasping, back to shore. Bob was already out of the water; Tom was right behind me.

I’d like to brag that we jumped back in and then jogged a few more miles barefoot, but this would be misleading.

Truth is, Sunday’s swim hadn’t exactly been impromptu. Hours earlier, Bob lit the stove of his wood-fired hot tub, which is situated only a few feet from shore. Seconds after immersing in the 37-degree lake, we were soaking in 106-degree tub water.

“Much more comfortable in here,” I sighed, idly watching the steam rise.

All in all, it was a gratifying completion of the monthly swims for 2019. It also occurs to me that it was the final one of the decade.

The satisfaction, though, will be short-lived. In a few more weeks, it’ll be time to start the cycle all over again.

Too bad there won’t be a hot tub waiting Jan. 1 at Esker Point Beach in Noank, where the New Year’s Day run transitions to the swim. Then again, I don’t think it would hold 200 or so participants.

Even so, it promises to be a fun day.

At least that’s what I tell myself every year.