The modern Mother's Day is celebrated on various days in many parts of the world, most commonly in May, though also in March, as a day to honour mothers and motherhood. In the United States it was nationally recognized as a holiday in 1914 after a campaign by Anna Jarvis. In some countries, it follows the old traditions of Mothering Sunday.
In 1912, Anna Jarvis trademarked the phrases "second Sunday in May" and "Mother's Day", and created the Mother's Day International Association. She was specific about the location of the apostrophe; it was to be a singular possessive, for each family to honour their mother, not a plural possessive commemorating all mothers in the world. This is also the spelling used by U.S. President Woodrow Wilson in the law making official the holiday in the U.S., by the U.S. Congress on bills, and by other U.S. presidents on their declarations.
As the US holiday was adopted by other countries and cultures, the date was changed to fit already existing celebrations honouring motherhood, like Mothering Sunday in the UK or the Orthodox celebration of Jesus in the temple in Greece. In some countries it was changed to dates that were significant to the majority religion, like the Virgin Mary day in Catholic countries, or the birthday of the daughter of the Prophet Muhammad in Islamic countries. Other countries changed it to historical dates, like Bolivia using the date of a certain battle where women participated.
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. (From: Wikipedia, license: CCA-SA)