Hinamatsuri (Doll's Day Festival) in Japan. It is one of the days in each year that mark a junction in the cycle of seasons. On a superficial level it celebrates "girlhood," the way Kodomo-no-hi (Children's Day Festival), on May 5, celebrates "boyhood," but it's actually more complicated than that. There are subtle regional and generational differences in how it is perceived and celebrated. You can read about it on Wikipedia.
This etegami shows two half-shells from the same clam. The smooth, white inside of one is painted with an image of the emperor, the other with the empress. Like other emperor/empress dolls painted or crafted according to ancient regional traditions, these would be displayed in the days leading up to the festival in every home that
has daughters. Clam shells are "a
symbol of a united and peaceful couple, because a pair of clam shells
fits perfectly, and no pair but the original pair can do so." (Wiki)