Barbara Cook at Avery Fisher Hall

Get out your calendar and circle January 8. You’re already busy? Nope, you’re not. Send your regrets — by phone, by email, by camel if need be — and get yourself tickets to Barbara Cook’s concert with the New York Philharmonic at Avery Fisher Hall. As thousands of concertgoers who attended Cook’s two sold-out shows this week can attest, this is a once-in-a-lifetime chance to hear a great, legendary artist at the peak of her interpretative powers — and backed by one of the world’s greatest symphony orchestras.

This concert was designed in celebration of Cook’s 80th birthday last month, and, as she has been for so many years, the lady is simply an inspiration in terms of stamina — both physical and vocal. She sailed through a demanding 90-minute, intermissionless set without a hint of fatigue, finishing up with a gorgeous, unamplified “Some Other Time.”

One of the many remarkable facets of the evening was Cook’s insistence on singing exactly what she wanted, when she wanted — and making us believe every instinct was right. Did it ultimately matter that none of the nearly 20 selections came from her signature Broadway roles — Cunegonde in Candide, Amalia in She Loves Me, Marian in The Music Man? Nope. (The Philharmonic, under Lee Musiker’s inspired direction, opened the concert with music from Candide.)

Did it seem odd for a soprano to tackle one of the greatest baritone songs in history, Rodgers & Hammerstein’s “This Nearly Was Mine” (from South Pacific)? Please. Cook brought so much feeling to the song, not to mention an unparalleled musicality, that it seemed a perfectly natural choice. Actually, Cook proved she could play either lead in that musical, with a particularly spirited rendition of Nellie Forbush’s anthem, “A Wonderful Guy.”

The patter was minimal — though Cook told one very funny story about being hired for a burlesque show at the age of 15 — but the vocal highlights were many: a brilliantly bittersweet “It Might As Well Be Spring,” a jazzy “Lover, Come Back to Me,” a triumphant, slightly melancholy “Smile,” and an insanely joyous “Accentuate the Positive,” to name a few.

Still, no surprise here, Cook shone brightest on the songs of her good friend, Stephen Sondheim. She brought a stunning gravity to “No More” — which had been ingeniously paired with an almost mournful version of Maxwell Anderson and Kurt Weill’s “Lost in the Stars” — and a maternal, worldly-wise warmth to “No One Is Alone.” But the concert’s absolute apex was a fiercely intelligent and absolutely heartbreaking reading of “I Wish I Could Forget You” (from Passion) that should tempt some savvy producer to get down on his knees and beg Cook to take a stab at Fosca.

As for Barbara Cook, she’s simply unforgettable.