What is Mother's Day?
Mother's Day is a celebration honoring mothers and celebrating motherhood, maternal bonds and the influence of mothers in society. It is celebrated on various days in many parts of the world, yet most commonly in March, April, or May. It complements Father's Day, the celebration honoring fathers, Parents' Day and Grandparents' Day.
Mother's Day around the world
In most countries Mother's Day is celebrated on the second Sunday in May, among them the USA, Canada, most European countries, Australia, New Zealand, India, China, Japan, the Philippines and South Africa. One notable exception to this rule are the UK and Ireland, which celebrate Mother's Day on the fourth Sunday in Lent. Most Arab countries celebrate Mother's Day on March 21st (vernal equinox), while most East European countries celebrate Mother's Day on March 8th (the same date as International Women's Day). Many French-speaking countries celebrate Mother's Day on the last Sunday of May (or, in the case of France, on the first Sunday of June in years in which the last Sunday of May is Pentecost). For a complete overview of the dates of Mother's Day around the world see Mother's Day on Wikipedia.

In most countries, Mother's Day is a recent observance derived from the holiday as it has evolved in America. When it was adopted by other countries and cultures, it was given different meanings, associated to different events (religious, historical or legendary), and celebrated on a different date or dates.

Some countries already had existing celebrations honoring motherhood, and their celebrations have adopted several external characteristics from the US holiday, like giving carnations and other presents to your own mother.

The extent of the celebrations varies greatly. In some countries, it is potentially offensive to one's mother not to mark Mother's Day. In others, it is a little-known festival celebrated mainly by immigrants, or covered by the media as a taste of foreign culture (compare the celebrations of Diwali in the UK and the United States).

Is it Mother's Day, Mothers' Day or Mothers Day?
Although the name of the event is usually understood as a plural possessive (i.e. "day belonging to mothers"), which would under normal English punctuation guidelines be spelled "Mothers' Day" (not "Mothers Day"), the most common spelling is "Mother's Day", as if it were a singular possessive (i.e. "day belonging to Mother").