Christmas beer?

Before 1975, that phrase might have meant a holiday carton adorned with sprigs of holly and jingle bells. Or maybe you splurged on some import in a green bottle to impress visiting relatives.

But the liquid inside the bottles was the same as the rest of the year.

That changed 39 winters ago, when Fritz Maytag decided to give his customers a special beer instead of a card. Maytag, then the owner of Anchor Brewing Company in San Francisco, debuted the first version of Our Special Ale (a.k.a. Anchor Christmas Ale): a hoppy pale ale that eventually was absorbed into the regular lineup as Liberty Ale.

Only 400 cases were bottled, recalls veteran Anchor brewmaster Mark Carpenter. “We had no neck labels back then, so Fritz got blank neck labels and wrote greetings to his friends.”

Last month, Anchor released the 40th iteration of Our Special Ale. It’s certainly the longest running winter release from an American brewery, and possibly the longest-running American seasonal beer of any kind. Like all of the ale’s vintages since 1987, this is a spiced ale, a unique recipe that’s brewed once then permanently retired. But this year’s formula is a bigger-than-normal departure from previous versions, confides Carpenter. “We decided to diverge a little bit, so we dropped some longstanding spices, added some new ones and changed where we add them, in the kettle rather than the cellar.”

More specific information is hard to come by, as the recipe for Anchor Christmas Ale is a closely guarded secret. That was the policy during Maytag’s tenure, and Skyy Vodka executives Keith Greggor and Tony Foglio, who bought the company in 2010, have decided not to tamper with tradition. “It makes it a little more fun,” concurs Carpenter.

Dave Alexander, former owner of the Brickskeller and R.F.D., hosted Carpenter on a number of occasions. He says of the Christmas Ale, “They would put the beer to a tasting panel and ask them to note any flavors recognized.” If a panelist correctly guessed one of the ingredients, “they felt that indicated a lack of balance.” And so they would add a little more of everything else to next year’s batch.

The 2014 Our Special Ale has a deep mahogany hue (it seems to have gotten progressively darker over the years) with a sharp, pleasant aroma reminiscent of incense. Notes of licorice and bittersweet chocolate mingle with a plummy fruitiness. There’s a hint of citrus in the finish and an aftertaste of nutmeg, maybe some clove.

The 2014 vintage seems a little drier and thinner than previous versions. It’s a complex beer and exceptionally drinkable, with whole-cone hops and highly roasted specialty malts offsetting the sweet spices.

A few facts have leaked out over the years. Carpenter says that the most frequently guessed ingredient is allspice, but claims he’s never used that in the beer. Although ginger seems to be the spice of the hour — Boston Beer and Hardwood Craft Brewery offer gingerbread stouts, and Harpoon Brewery just introduced a ginger-flavored version of its UFO wheat — it has never found its way into Our Special Ale, either, asserts Carpenter.

Carpenter admits that in 2004 he used the Biblical spice frankincense, but he quickly dropped it. “I was an altar boy when I was a young man, and it reminded me too much of church,” he explained.

It’s not that Our Special Ale is any way anti-religious. In fact, it unabashedly refers to itself as “Christmas Ale” when most of its competitors label themselves as “holiday” or “winter” brews to avoid being tossed out with the tattered wrapping on Dec. 26.

“A lot of our distributors say, ‘You should call it a holiday ale, you can sell it for longer,’ ” says Carpenter. But Anchor times the annual release (the brewery this year made about 7,000 barrels, or 98,000 cases, he estimates) so it sells out in a two-month window between early November and the start of the New Year. And if it doesn’t? You can do things with leftover beer that you can’t do with leftover turkey. Unsold Christmas Ale is transported to Anchor Distilling Company, where it’s turned into a 90-proof beer schnapps called Christmas Spirit (available only in California, unfortunately).

The ultimate test for Our Special Ale, Carpenter believes, comes at the dinner table. “Does it really fit in with your holiday meal? Then you know it’s a success.” He found this year’s version a perfect complement to his wife’s turkey dressing, which he describes as “simply bread, celery, onion, mushrooms, poultry seasoning and lots of butter.”

Every year, the label for Anchor Christmas Ale bears the image of a different tree. This year’s species is the giant sequoia, drawn by graphic designer James Stitt. Capable of growing over 300 feet tall and living 3,500 years, the sequoia is a symbol of permanence. What about the beer?

Maytag liked to call his Christmas beer a “moving target,” believing that the constantly changing recipe would discourage imitators. But the shelves in the beer aisle today are groaning with seasonal releases. You’re almost a Scrooge if you offer just one. The holiday assortment from Flying Dog Brewery in Frederick, Md., contains four new beers, including an Oatmeal Raisin Stout and a Hazelnut Scotch-Style Ale, inspired by cookies from Otterbein’s Bakery in Baltimore. Boston Beer Co. has three spiced beers in its Samuel Adams winter variety pack (not including the gingerbread stout), and was readying Samuel Adams Cold Snap (a white ale with coriander, orange peel and plum) for a January launch.

Even Anchor, for the first time, has a second winter seasonal on the market: a dark wheat ale called Anchor Winter Wheat.

Might Anchor be tempted to retool its Our Special Ale? “I’m semi-retired, and it’s possible that a new brewer would want to do something different,” concedes Carpenter. “But it has such a popular following that it would disappoint a lot of people.”

Kitsock is the editor of Mid-Atlantic Brewing News.

A previous version of this column misspelled the name of Skyy Vodka.