If American Players Theatre has a royal court -- and most repertory theaters do -- then Sarah Day is the queen.

She has been a member of the acting company since 1986, often specializing in the role of dotty old mothers. She's currently starring in the role of Lady Bracknell in “The Importance of Being Earnest,” and she generally adds a light and comforting touch to the stage.

This year Day is also the star and only cast member of Joan Didion's memoir-play, “The Year of Magical Thinking.”

I believe I have been in the audience of almost every production Day has been in for at least the past 25 years, and I have no hesitation in asserting that this is flat-out the best thing she has ever done.

The story is that of Didion, whose husband, John Gregory Dunne, like Didion, a famous writer, dropped dead of cardiac arrest while sitting at the dinner table of their Manhattan apartment. The two had just returned from the hospital where their daughter, Quintana Roo Dunne, was in a coma being treated for toxic shock.

Quintana recovers, but on a trip to Los Angeles, she collapses from bleeding in the brain. She then recovers from that and dies a year later in New York of pancreatitis. All in all, it is a hard year for Didion, to say nothing of her husband and daughter.

In most of Day's plays, she makes the roles she is acting conform to the character she has developed over the years, though that wasn't true in last season's productions of “All My Sons." In this production, Day disappears into Didion's persona. She doesn't look like Sarah Day. She doesn't sound like Sarah Day. She appears to be Joan Didion. In fact, the real Didion might be a disappointment.

And she does this without stopping for almost two hours, which has to be a tremendous physical exertion.

But, in addition to Day's virtuoso performance, I think we have to say something about the director, Brenda DeVita.

Directors tend to be invisible. They're not on the stage. They get to write little spiels for the programs explaining their vision, which often has little to do with what the audience actually experiences. So we don't really know what DeVita contributed to this production.

Except this: Sarah Day is a great actress. She has been a great actress for decades. Yet, somehow, Brenda DeVita pulled something extra out of her, some depth of experience, some empathy that turned Day into Didion and Didion into every widow.

“The Year of Magical Thinking” is a remarkable experience. If you haven't seen it, make sure you do.