It's always on a Sunday. It's the 4th Sunday of Lent.
Once upon a time it was called Mothering Sunday, and church services were held to celebrate it. My mother always insisted it be called Mothering Sunday and that Mother's Day was an Americanism. Do you know how hard it was to find a Mothering Sunday card?
Originally it was a day people went to their home or 'mother' church which was the main church of the parish. On the 4th Sunday of Lent it was considered important to worship in the main church of the parish instead of the local chapel. Then it became the day that child servants were given the day off, and they would go home to visit their mothers.