Mothering Day, or Mid-Lent Sunday, is also known as Laetare and, as noted is a slight relaxation of the Lenten rules. (By the way, no relaxation of Lenten austerity is necessary on a Sunday, since Sundays are always holidays, and not part of Lent. Lent is 40 days in length, but if you count from Ash Wednesday to Holy Saturday, you don't come to 40 if you include the Sundays. Every Sunday is a "little Easter.") Laetare has a counterpart in the Advent season, Gaudete, which is the 3rd Sunday in Advent.
In Catholic and Anglo-Catholic churches, the correct liturgical color for Gaudete and Laetare is rose, a relaxation of the violet which is used for the rest of Advent and Lent, in the old Roman use. Rose vestments and their matching altar frontals, pulpit falls and lectern markers are rarely seen, since liturgical vestments are costly and few churches will spend the money for a set used only twice a year unless some benefactor donates them.
I don't know where Laetare gets its name, but probably similarly to Gaudete, which takes its name from the proper Epistle for that Sunday, which begins, in the Latin Sarum Use, "Gaudete semper in Domino", "Rejoice in the Lord always".
