FYI: I put a monthly list of festivals of Gods for Rome and Mesopotamia in separate posts.
For Romans, May (Maius) is sacred to Maia, the Goddess of the Growth of Living Things. As the Mother of Mercury, She is also honored with Him at the Mercuralia on May 15. On May 1st, Maia’s festival day and on the 15th, a priest of Vulcan (God of Fire) will sacrifice a pregnant sow to Her. Maia is his consort since Vulcan (Volcanus) is also the God who ripens the earth with his inner warmth. Modern Roman Polytheists will offer burnt pork to Maia.
May is also a gloomy month since the Dead roam freely at this time. The Lemuria is to ensure that the Dead are placated and do not trouble the living. Meanwhile, the Rosalia focused on placing roses and violets on graves.
The Days of the Dead
The major focus of this month is the Lemuria, the Roman Days of the Dead (May 9, 11, and 13). On these days, the Lemures (Larvae) seek out the living to have them give the Larvae proper burials. The Lemures also want people to make offerings in their memory to the Gods of the Dead. Meanwhile, the living do certain rites to ensure that Larvae not harm them or their families. (The Larvae could be considered the “Undead.”)
Until the 8th Century, May 13 was All Saints’ Day for Christians. During the 730s, Pope Gregory III changed the feast date to November 1. He wanted to accommodate the Celtic Christians, who had grown in numbers. Meanwhile, Roman Lemuria can be considered the Roman equivalent of Halloween, All Saints’ Day, and All Souls’ Day.
For Romans, Mercury is the God of Commerce, Merchants, and Thieves. On May 15, merchants would bless themselves and their wares from his sacred well, which was located outside of the Sacred Boundary (Pomerium) of Rome. Modern Roman Polytheists will use water from local streams to bless their local banks and stores.
Julius Caesar noted that Mercury was the most popular God in the Celtic and Germanic regions closest to Roman territories. These peoples regarded Mercury to be the inventor of the arts. In Celtic areas, He was frequently accompanied by Rosmerta, Celtic Goddess of Abundance and Prosperity.
On May 23, the Rosalia (dies rosationis (the day of the rose adornment)) is held. This was originally a military rite to honor the fallen. It later became a ritual to honor all the dead, with roses placed on graves. For the Rosalia, I would suggest going to a battlefield or military cemetery, if possible.
At the end of May, people would walk the perimeters of their fields bringing offerings of milk, honey and wine. Ancient Romans herded a boar, ram, and a bull around the boundaries, and then sacrificed them. Modern Roman Polytheists offer meats from the store, and ask for the blessings of Mars and Ceres on the crops.