The modern Mother's Day is celebrated on various days in many parts of the world, most commonly in May or in March/April, as a day to honor mothers and motherhood. In the UK and Ireland, it follows the old traditions of Mothering Sunday, celebrated in March/April.
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, 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.