All of nature awakened to new life in springtime is presented to honor Mary, who is herself "a rose of Sharon, a lily of the valleys" (Song of Songs 2,1). This form of devotion was influence and furthered, for example, in Treatise on True Devotion to Mary by Louis de Montfort, who, among other things, counted the decoration of Marian altars a chief exercise of Marian devotion.
May Altar--Specially Decorated Altar in Churches
When erecting a May
altar in a church, one distinguishes between the special decoration
of an existing Marian altar, the erection of an altar set up
specifically to serve this May devotion, or the transformation of the
main altar into a May altar. The Handbook of Church Rituals
(Regensburg 1846) notes under May altar that these devotions be held
at an altar dedicated to the Blessed Virgin and decorated "to the
full." If there isn't any [altar dedicated to Mary], another
altar is to be set up and furnished with a picture or a statue of
Mary. In Strasbourg, in 1855 for the first time, a
special "Mother of God altar" was set up before the chancel.
A side altar of this type was drawn into the celebration in that the blessing frequently was given from this altar. By carrying the Blessed Sacrament from the main altar, the precedence of the main altar was clearly visible.
Little May Altar--For the Home
With the development of May altars in churches, the custom spread to set up this type of "altar" also in the home. The authors of both private publications and of official publications refer to this practice, encourage them, or assume that there are such. While some devotional books encourage the user to decorate an image of Mary found there and to pray there--a custom "that belongs anyway in every good Catholic home"--others depict the "prayer room" as "a shrine dedicated to Mary."
[Source literature: J. B. Metzler, "Die Maiandacht im Familienheiligtum," In: Sendbote des göttlichen Herzens Jesu 45 (1909) 137-140; also "Am Vorabend des Maimonats, In: Präsides-Korrespondenz für marian. Kongregationen 17 (1923) 70-73. K. Küppers, Marienfrömmigkeit zwischen Barock und Industriezeitalter, 1987 (Lit.)]"
Author: Professor Kurt Küppers, Augsburg. Marienlexikon, Vol 4, p. 243-244