Tea parties were a real thing back in the day. Starting with Victorians, the afternoon tea was a daily event in any respectable well-off family. The fashion quickly spread all over Europe, and reached the US, during its Gilded Age (the period from the 1870s to about 1900). Here affluent families used teas as debutante parties.
Mary Cassatt was a great admirer of Edgar Degas. Both artists had a long period of collaboration. Cassatt said: "How well I remember seeing for the first-time Degas' pastels in the window of a picture dealer. I would flatten my nose against the window and absorb all I could of his art." Degas would help Cassatt get models for her work. Here one of those models was the woman sitting with Cassatt’s sister Lydia.