Mothering Sunday is not a fixed day because it is always the middle Sunday in Lent (which lasts from Ash Wednesday to the day before Easter Sunday). During the sixteenth century, people returned to their mother church for a service to be held on Laetare Sunday. This was either a large local church, or more often the nearest Cathedral. Anyone who did this was commonly said to have gone "a-mothering", although whether this preceded the term Mothering Sunday is unclear. In later times, Mothering Sunday became a day when domestic servants were given a day off to visit their mothers and other family members. It was often the only time that whole families could gather together, since on other days they were prevented by conflicting working hours.
[We tend to forget the background so thought I would just add a little information]
We moved this from Mum and Dad's garden when we sold the house - and it is the first time it has flowered. I took this yesterday afternoon when we had a break in the cloudy sky and the light improved a little. So for all those who go a-mothering, remember your Mum or celebrate being a Mum today, have a super day.