Dea Dia is a very ancient Roman Goddess of the plowed field Who is concerned with the fertility of the earth and the growth of the planted crops, especially grain. Her worship dates back to early times, and is of either Latin or Sabine origin; She is closely connected with the Arval Brethren, the ancient brotherhood of a dozen priests whose job it was to offer sacrifices for the robust health of the fields and crops.

The name Dea Dia has been described as more a title than a proper name: while Dea just means "Goddess"; Dia seems to share the same root as the Goddess-name Diana, both having their origins in words for "light" and "sky". The ideas of "Deity" and "Light" seem to be conceptually related in Indo-European thought; Latin divum can mean both "God" and "Sky"; and the archaic form of the Sky-God Jupiter's name is Dies Pater, literally "Daylight Father". Dea Dia is surmised by some to have been the consort of an early form of Jupiter, for another archaic name for Him is Dius, the masculine form of Dia. Dea Dia would then mean "the Celestial Goddess"; and the similarity of Her two names provides emphasis and attests to Her power and holiness, much like the doubled name of Maia Maiestas. And like that of the Bona Dea, Dea Dia's original name may have been forbidden to be spoken due to its sacred nature.

If Dea Dia was originally a Sky-Goddess, then Her role in agriculture would be as the Goddess Whose light causes the crops to grow and mature. Her main festival in May was celebrated by the Arval Brethren, who attended to Her rites. May of course is a time of year in which the days are getting longer and the light is increasing; and Her lengthening daylight was acknowledged and honored for its part in causing the crops to ripen. Her lesser festival was in December, which in the mild climate of central Italy is a time when crops are sown; and the May festival came shortly before the first harvest, usually in June.

Dea Dia had a sacred grove called the Lucus Deae Diae on the Via Portuensis (or the Via Campania, as the two roads run on top of each other for some distance before diverging), about 5 miles south of Rome. This grove bears evidence of being used since at least the 3rd century BCE: here Her festival was officiated over by the Arval priests, and here that brotherhood held their meetings and attended to their business. The temple in Her grove was of a circular plan, built high on a platform; it was made of marble, and the outside surface was engraved with the yearly records of the priests, fragments of which survive and which provide us with a lot of information about Her cult and the practices of the Arvals. Remains of a bath-house have also been found there, which may have been used by the priests during part of the Goddess's festival, and a circus or race-track was also built on the site.

The festival of Dea Dia is usually said to have been celebrated over three days in May, though some sources give a date in June. As it was a moveable feast, perhaps this reflects two traditional dates—the first set of which are given as May 27th, 29th, and 30th, and the second as June 17th, 19th, and 20th—that were chosen from. Generally Roman holidays that stretched over more than one day were celebrated on the odd-numbered days, which were considered lucky (the even numbers being held unlucky), so the inclusion of the 30th of May and the 20th of June in Her holiday is a little surprising. It is however rather more likely that the three days of Her May festival were determined by the date when the crops had been sown, usually in December. In January (on either the 7th or 11th) the Magister, the elected leader of the Arval Brethren, announced the date of the coming year's festival from the steps of the (smaller) temple to Concordia on the Capitoline Hill in Rome.

On the first day of Her festival the Arval Priests, wearing their traditional crowns of wheat-sheaves, met at the home of their Magister in Rome before dawn and made offerings of fruit, incense and wine to Dea Dia as the sun rose (which again connects Her with the idea of strengthening light). After these offerings were given Her statue was anointed; the priests then ritually bathed, the Magister changing out of his praetexta (a specific type of purple-bordered toga only Magisters were allowed to wear) into white clothing for dinner. After dinner was eaten, but before dessert was served, the Magister again offered fruit, incense and wine to the Goddess.

The second day of Her festival was considered the main day of the celebration and included public rites at Her grove and temple, with offerings given to ensure the fertility of the earth and crops. The festival again began in the morning, when the Magister sacrificed two young pigs and a white cow to Dea Dia. The Magister was then joined by the rest of the Brethren, who prepared a breakfast from the sacrificed animals. Then then sacrificed a lamb and placed earthenware pots or wine jars on the altar (which were later destroyed so as not to be used again); then they offered wheat (collected from the crowd outside the temple), pastries, and vegetables (including turnips), and shared a special bread called panes laureati, or bread made with laurel (bay) leaves, which come to think of it I might just have a recipe for. The temple was then cleared and the ritual dance begun, in which the famous hymn of the Arval Brethren was chanted:

Nos, Lares, iuvate
Neve luem ruinam sinas incurrere in plures.
Satur esto, fere Mars!
In limen insili! Sta! Verbera!
Semones alterni advocate cunctos.
Nos, Mamers, iuvato!
Triumpe!

Which translates to:

Both the Lares and the Semones invoked in this hymn were protective spirits or minor Deities, the Semones being especially connected with the earth and the sowing of crops. The Lares were guardian Deities who likely had their origins in the spirits of the dead who were buried in the earth; later They seem to have been associated with farmland and fields, and came to be adopted as protective spirits of the household. They are also connected to Dea Dia, as She was identified with Acca Larentia, mother of the Lares, whose twelve sons were said to have made up the first Arval priesthood.

When the dancing was finished, the temple doors were opened and the Brethren gave food, money, and roses to the crowd. Horse races were then held in the circus, and included races with two- and four-horse chariots as well as performances by the desultores, acrobats who specialized in vaulting between running horses. The Magister, of course, presided over these games and handed out the prizes. Also on this day the Arval Brethren held their elections, and on the following December 17th (probably the date of Dea Dia's lesser festival) the new Magister was inducted into office. (December 17th was also the first day of the Saturnalia, the great winter festival that was dedicated to Saturn, Roman God of sowing and crops. And on the 19th of December the Opalia was celebrated, dedicated to Ops, the wife of Saturn; She was also a Goddess of the Fields, and in fact was often associated with the Dea Dia.) At the conclusion of this day, the Brethren then went back to the Magister's house for supper.

The third day of the festival was again celebrated at the house of the Magister, and its rituals were similar to those followed on the first day.

Dea Dia has been linked to many other Goddesses with agricultural connections such as Ceres, Ops, and Acca Larentia, whose twelve sons were traditionally the first Arval priests. Again in connection with the Lares, Dea Dia was linked to Mana Genita, mother of the Manes or spirits of the dead, or Larunda, an Underworld Goddess. Fors Fortuna had a temple within the Lucus Deae Diae; because of this the two have been linked as well.