The modern Mother's Day holiday was created by Anna Jarvis as a day for each family to honor its mother, and it's now celebrated on various days in many places around the world. It complements Father's Day, the celebration honoring fathers.
This holiday is relatively modern, being created at the start of the 20th century, and should not be confused with the early pagan and Christian traditions honoring mothers, or with the 16th century celebration of Mothering Sunday, which is also known as Mother's Day in the UK.
In most countries the Mother's Day celebration is a recent holiday derived from the original US celebration. Exceptions are, for example, the Mothering Sunday holiday in the UK. 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 addition to Mother's Day, International Women's Day is celebrated in many countries on March 8.