[Quoth]

In the UK and Ireland "Mothering Sunday" IS called Mother's Day and has been for a long time. Most people in UK and Ireland wouldn't even know it used to be called Mothering Sunday. For most people it's purely secular, though the origin is Anglican in the UK and Ireland. Catholics do send Mother's Day cards and flowers. The Wikipedia articles are biased. Other Countries unrelated to the days of the British Empire and GB Colonies have their own Mother's Day also unrelated to the USA Mother's day or UK Mother's Day, once called Mothering Sunday which was already pretty secular in the 19th C, more about people in Service visiting home than going to the "Mother Church" even in the 18th C.

As a kid, I never realised it was movable and in Lent. One theory also about Lent is St. Patrick's Day which is often in Lent. It's a Feast Day, being he's one of the three Irish Patron Saints, though the most recent and not Irish*. A Feast Day during the Lenten Fast, hence the popularity of it especially from the 17th C.

[* But England's St. George was probably Romano-Turkish and was never in England, unlike Patrick who was in Ireland. Though the Shamrock story is fake as is banishing snakes]