This coming Sunday is Mother's Day in the USA. Although it's often looked on as a "Hallmark holiday" it actually has a variety of definitions and derivations depending upon the country in which it is being celebrated. The second Sunday in May is the most popular date with over 60 countries participating.
In terms of Google traffic the next most popular date is 'Mothering Sunday', a related but much older festival. This is celebrated in the UK, Ireland and Nigeria and is an unusual secular example of a movable feast. Based originally on Laetare Sunday in the Christian Liturgical calendar, the date of observation is the fourth Sunday in Lent, which inevitably ties it to the annular dating of Easter.
Many countries celebrate International Women's Day annually on March 8th. Mongolia observes this day as well as 'The Mothers and Children's Day' on June 1st. Other unusual dates include an observation in the Middle East at the vernal equinox and observations in Israel and Iran which are dated in the Jewish and Iranian calendars respectively.