It turns out that, according to Wikipedia, "the celebration of Mother's Day began in the United States in the early 20th century; it is not related to the many celebrations of mothers and motherhood that have occurred throughout the world over thousands of years, such as the Greek cult to Cybele, the Roman festival of Hilaria, or the Christian Mothering Sunday celebration".
The Wikipedia entry for Mothering Sunday sheds further light (or, dare I say it, perhaps confusion?) on the situation. Note the clear 'instruction' on that page: "Not to be confused with Mother's Day".
