Featuring lovely fashion illustrations from the 1920s by the famed French artist-designer George Barbier, this whimsical 16-month wall calendar is sure to delight.   From a fetching young woman musing at her vanity to a couple dancing the Charleston in the moonlight to a group of ladies frolicking on the beach, the 13 charming illustrations featured in this calendar capture the bon vivant's spirit of the roaring 1920s. All are from Barbier's famed book Le Bonheur du jour, or Happiness of the Day, a copy of which is the collection of the Met's Costume Institute library.   The Costume Institute's collection of more than 35,000 costumes and accessories represents 5 continents and 7 centuries of fashionable dress, regional costumes, and accessories for men, women, and children from the fifteenth century to the present.