The original is Mothering Sunday, the third Sunday in Lent, when people took the opportunity to visit the 'mother cathedral' or church. Because Lent is a moveable feast, dependant on Passover and lunar calendars, so is Mothering Sunday.

Various countries decided to pick a particular date and stick with it. That got called Mothers Day and so the name started to be used in the UK. We have Mothering Sunday rather than Mothers Day but most of the cards and gifts have Mothers Day on them, so that's what it's become.

But as a mother, thanks.

LTS