Mother's Day is celebrated on various days in many parts of the world, most commonly in May, but also in February, march, and April in some places. In the United States it was nationally recognized as a holiday (on the second Sunday in May) in 1914 after a multi-year campaign by Anna Jarvis (she died in 1905 and her daughter Anna Marie carried on the campaign). We use the singular (mother's) as opposed to the plural (mothers') to commemorate family mothers vs. all the mothers in the world.
As the US holiday was adopted by other countries and cultures, the date was changed to fit already existing celebrations honoring 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, such as the Virgin Mary day in Catholic countries, or the birthday of the daughter of the Prophet Muhammad in Islamic countries. Bolivia uses the date of a certain battle where women participated. Many Arab countries celebrate on March 21, the day of the vernal Equinox.
Traditionally carnations represent Mother's Day. Many religious services copied the custom of giving away carnations or wearing a carnation on Mother's Day. Florists promoted wearing a red carnation if your mother was living, or a white one if she was dead and this has remained popular.