Birth of the Virgin, Murillo, 1660
Like the May devotions themselves, the custom to set up a May altar stems from southern European countries. A report from France in 1842 speaks of Our Lady's altar in May showing off in rich splendor, while the families also erected and decorated small home altars.
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.
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: revised from University of Dayton
Creating an Altar for Honoring Mary
Choose a room in your home to set up an altar, perhaps just a corner or a table in a hall. Use a narrow table or a small chest of drawers, covered with a cloth, or not.
Use an image of Mary for your centerpiece -- a small statue if you have one, or a picture of Mary. If you don't have a statue in your home, consider making a purchase -- even at a thrift store. A rosary can also be placed on the altar.
It's nice to have a chair nearby if you want to do your daily prayer or reading.
Any flowers can be used on a home altar, to reflect on the beauty of the coming summer. Many flowers throughout history have been associated with the VirginMary, so it's nice to use them.
Candles can be another addition to your special area, and could be lit during prayer times (unless you have tiny children). When the candle is lit, children can offer their prayers and extinguish the candle when the prayer session ends.