The fourth Sunday in Lent (Mid-Lent) derives its Latin name from the first word of the Mass text, "Laetare Jerusalem" (Rejoice, O Jerusalem). It is a day of joy within the mourning season. The altars may be decorated with flowers, organ playing is permitted, and the priests may wear rose-colored vestments instead of purple. The reason for such display of joy is explained in a sermon by Pope Innocent III (1216):
"On this Sunday, which marks the middle of Lent, a measure of consoling relaxation is provided, so that the faithful may not break down under the severe strain of Lenten fast but may continue to bear the restrictions with a refreshed and easier heart."As a symbol of this joy the popes used to carry a golden rose in their right hand when returning from the celebration of Mass. Pope Leo IX (1051) calls this custom an "ancient institution." Originally it was a single rose of natural size, but since the fifteenth century it has consisted of a cluster or branch of roses wrought of pure gold and set with precious stones in brilliant workmanship by famous artists. The popes bless it every year, and often they confer it upon churches, shrines, cities, or distinguished persons as a token of esteem and paternal affection. In case of such a bestowal, a new rose is made during the subsequent year. The meaning and symbolism of the golden rose is expressed in the prayer of blessing. It represents Christ in the shining splendor of His majesty, the "flower sprung from the root of Jesse." From this ecclesiastical custom Laetare Sunday acquired its German name, Rosensonntag (Sunday of the Rose).
In England a charming tradition developed toward the end of the Middle Ages. On Laetare or Mid-Lent Sunday, boys and girls who lived away from home (as apprentices, servants, etc.) were given permission to go home to visit their mother church, in which they were baptized or had been brought up. They always carried with them gifts to put on the altar. The original reason for this was because the first words of the Mass, "Laetare Jerusalem," were considered in medieval times to be addressed to the "second Jerusalem" (the Church). And as the Jews called Jerusalem "Mother Jerusalem," so the Christians later called the church which gave them spiritual birth in baptism, "Mother Church." It was also the custom for the boys and girls to visit their own mother on the same day. They brought her flowers and simnel cakes (a rich plum cake; from simila, fine flour) and would do all the housework for her. This old custom still survives in certain parts of England, and the cakes are sold in London as well as provincial towns. Hence the name "Mothering Sunday".
~From Lenten Customs