Mercuralia falls on the Ides of May, which means the middle of the month or the 15th of May. This is a festival to honour the Roman God Mercury who became known as Hermes in the Greek Tradition to follow.
This festival of Mercury was predominantly celebrated by business men and merchants, for Mercury was seen to be the primary god of commerce and therefore merchants. On May 15 merchants would sprinkle their heads, their ships and merchandise, and their businesses with water taken from a sacred well from the city of Porta Capena.
There are many who still celebrate Mercuralia in modern days. There are those who honour Mercury and or Hermes on this day.
Most devise rituals of their own to honour the gods.
There are many elements traditional around this god that are interesting to explore, and many from different paths are familiar with symbolic items such as the staff he wields, the Caduceus...
Homeric Hymn to Hermes:
I sing about Hermes, the Cyllenian slayer of Argus, lord of Mt. Cyllene and Arcadia rich in flocks, the messenger of the gods and bringer of luck, whom Maia, the daughter of Atlas, bore, after uniting in love with Zeus.
She in her modesty shunned the company of the blessed gods and lived in a shadowy cave; here the son of Cronus used to make love to this nymph of the beautiful hair in the dark of night, without the knowledge of immortal gods and mortal humans, when sweet sleep held white-armed Hera fast.
But when the will of Zeus had been accomplished and her tenth month was fixed in the heavens, she brought forth to the light a child, and a remarkable thing was accomplished; for the child whom she bore was devious, winning in his cleverness, a robber, a driver of cattle, a guide of dreams, a spy in the night, a watcher at the door, who soon was about to manifest renowned deeds among the immortal gods.
Maia bore him on the fourth day of the month. He was born at dawn, by midday he was playing the lyre, and in the evening he stole the cattle of far-shooting Apollo.
So hail to you, son of Zeus and Maia. Hail, Hermes, guide and giver of grace and other good things
(to be repeated 3 times)
The following video is a modern day Pagan Temple's Celebration of Mercuralia: