Festivals celebrating motherhood have been around for thousands of years. The Romans had a festival to celebrate Cybele, the Earth Mother. The Greeks also honoured Gaia who was also the mother of the earth and can be viewed as the equivalent of Mother Nature.
So the concept of an earth mother is thousands of years old, but by the time Christianity had swept across Europe the attention switched to the Virgin Mary. Thus the celebration centred on the Christian festival of Lent and Mothering Sunday is now celebrated on the fourth Sunday in Lent. There is also the concept of the mother church where parishioners of the smaller village churches would visit the main church or cathedral. This would require a day off work especially for the servant girls and so the tradition of a day of rest for mothers evolved.
Other names for this day are Simnel Sunday or Rose Sunday. Simnel Sunday was named after the practice of baking simnel cakes and Rose Sunday refers to the tradition of posies of flowers being distributed to mothers on this day.