A week before Christmas, in the midst of a Midtown Manhattan decked in its holiday finery, Patti Smith, New York City’s punk poet laureate, gave an unannounced performance before a fast-gathering crowd of several hundred in the lobby of the Museum of Modern Art.

Ms. Smith, who will turn 70 this month, was in a congenial and casual mood. She wore a black blazer and jeans, sang four songs, and tested out some new glasses by reading about 10 poems.

Several times, she interrupted herself mid-reading to tell a funny story or to chuckle at herself for forgetting lyrics — again. (Ms. Smith, overcome with emotion, also stopped midway through a rendition of “A Hard Rain’s a-Gonna Fall” recently at the Nobel Prize award ceremony as she accepted Bob Dylan’s literature prize.)

The performance was dedicated to the French novelist and playwright Jean Genet, who was born on Dec. 19, 1910, and was an idol of Ms. Smith’s. She has performed at the museum on or around his birthday nearly every year since 2010. But she also jokingly made note of some other recent birthdays.