Wikipedia, the source of all alleged facts, has some interesting notes on Mothers Day.

Different countries celebrate Mother’s Day on various days of the year because the day has a number of different origins.

One school of thought claims this day emerged from a custom of mother worship in ancient Greece, which kept a festival to Cybele, a great mother of Greek gods. This festival was held around the Vernal Equinox around Asia Minor and eventually in Rome itself from the Ides of March (15 March) to 18 March.

The ancient Romans also had another holiday, Matronalia, that was dedicated to Juno, though mothers were usually given gifts on this day.

In some countries Mother’s Day began not as a celebration for individual mothers but rather for Christians

…Mothering Sunday, also called “Mothers’ Day” in the United Kingdom and Ireland falls on the fourth Sunday of Lent (exactly three weeks before Easter Sunday). It is believed to have originated from the 16th century Christian practice of visiting one’s mother church annually…

…In the United States, Mother’s Day was loosely inspired by the British day and was imported by social activist Julia Ward Howe after the American Civil War. However, it was intended as a call to unite women against war… Her idea was influenced by Ann Jarvis, a young Appalachian homemaker who, starting in 1858, had attempted to improve sanitation through what she called Mothers’ Work Days…

…Nine years after the first official Mother’s Day, commercialization of the U.S. holiday became so rampant that Anna Jarvis [daughter of Ann Jarvis] herself became a major opponent of what the holiday had become. Mother’s Day continues to this day to be one of the most commercially successful U.S. occasions…