There are a number of explanations for the origins of Mothering Sunday, some going back to ancient Rome, but in its modern form it probably had little to do with mums at all.
Before the Reformation it was customary for parishioners to attend a service at their nearest Cathedral or ‘Mother Church’ on the fourth Sunday in Lent - hence Mothering Sunday. After Henry VIII decided to dispense Catherine of Aragon and the Catholic Church it became a holiday when rural servants and labourers would go to visit their families, perhaps taking with them a simnel cake as a treat (simnel cake is a boiled cake containing plums and coloured yellow with saffron). The word ‘simnel’ refers to the top grade of flour made in the middle-ages and only available to wealthy folk - so mum was being really spoilt when she was given this gourmet treat.
Mother's Day, Mothering Sunday
Incidentally, Mothering Sunday and Mother’s Day have quite different origins. Mother’s Day is an American invention started by Miss Anna Jarvis of Philadelphia and established by Congress in 1914. American mums will have a while to wait before their special day which is observed throughout the US on the second Sunday in May.