The second Sunday in May is Mother’s Day in the U.S. Over 40 other countries celebrate it, though sometimes on different dates. See the Wikipedia article for a list by country.
Origin of Mother’s Day in the U.S.
The Credo Reference topic page on Mother’s Day has several encyclopedia articles about its origin.
This featured article from the Encyclopedia of Motherhood explains:
“Andrews Methodist Church in Grafton, West Virginia, hosted the first official American Mother’s Day service on the morning of May 10, 1908. The Wannamaker Auditorium in Philadelphia, Pennsylvania, hosted a larger Mother’s Day service, with 15,000 people in attendance, that same afternoon. Both events were organized by Anna Jarvis, who was inspired to establish a maternal memorial day in honor of her mother. By 1911, every state observed Mother’s Day. However, it did not become a national holiday until 1914, when a Congressional resolution and Woodrow Wilson’s presidential proclamation established the second Sunday in May as Mother’s Day and called for the display of the American flag on all government buildings and individual homes.”
Jarvis encouraged the use of the white carnation, her mother’s favorite flower, as a symbol of Mother’s Day.
Take a look at that article and others on the topic page to explore the precursors to Mother’s Day in the U.S., celebrations of mothers in other cultures that predate the U.S. Mother’s Day, and details on how Mother’s Day became so commercialized (which, surprise, is what led to Father’s Day!)