
This image appears to have been taken at an event Marilyn attended on April 26, 1955: The Banshee Luncheon. The Banshees were a New York City Luncheon group made up mainly of artists, writers, and editors. Additional images of Marilyn at this event can be seen here.
Interesting point of note, in this rarely seen image, Marilyn is wearing her mink fur collar, which today is part of the Marilyn Monroe Collection. For more on this collection piece, click here.