Mercury (Mercurius) was not originally a Roman God. However, He was assimilated so early that He became one of the Di Consentes (The Twelve Great Gods). Mercury came to Rome via the grain trade with Sicily, which was then a part of the Magna Graecae (Greater Greece). The Romans first considered Hermes, the Greek God, to be the God of the Grain Trade. Later as Mercurius, He became the God of Trade and Merchants. However, Cicero that one of Hermes’ aspects – the Messenger of the Gods – was carried over from the Greeks.
In 495 BCE, Mercury’s temple was built outside the Pomerium (Sacred Boundary of Rome). The Mercuralia, his major festival, held on the Ides of May, the day when his temple was dedicated. Since his temple is located halfway between the temples of the Capitoline Triad of the patricians and the Aventine Triad of the plebeians, Mercury also became the Mediator Between Social Classes.
On May 15, the merchants would make offerings to Mercury and His Mother, Maia. With a bough of laurel, they would bless themselves and their wares from the aqua Mercurii, the water beside the temple. (Modern Romans bless their financial instruments (checkbooks, etc) and their banks.)
Prayers to Mercury:
From: Q. Horatius Flaccus
It is well. Nothing more ample do I pray, O Maia’s son, save that You will make these my gifts last throughout my life. May You, Mercurius, make plump the riches of my house and all else there, spare my natural talents in any case, and as usual, may You remain the primary guardian over me. – Datura 2.6.4-5;2.6.14-15
“O Mercury whether I have falsely called You to bear witness in the past, or deceitfully called upon Jupiter not to hear my empty promises, or if there is some other god or goddess that I knowingly deceived, wash away my past perjuries to make when the new day dawns, and make the gods be indifferent to my lies. Grant that I may profit, grant joy in making a profit, grant I many enjoy once more swindling my customers with deceitful words.” Fasti 5.691-90