Neptune was the god of the sea (as Neptune Oceanus) and of all waters for Romans, but he was not among Rome's most revered and powerful gods. Little is known of his origin: when he was first introduced in Rome, he already had acquired all the characteristics of the Greek Poseidon. Despite the fact that his cult grew after his equation with Poseidon, Neptune was far less popular among Roman sailors than Poseidon was with Greek mariners.
The Neptunalia was originally mostly a private affair concerned with Neptune's protection of fresh agricultural water. It took on greater significance as a public observance after Marcus Vipsanius Agrippa, Augustus's military Chief of Staff and his Admiral, dedicated a temple and porticus to Neptune after his naval victories over Marc Antony at Azio and, shortly thereafter, at the decisive battle of Actium, in 31 BC. Agrippa's sanctuary was in the Campus Martius between his Pantheon and baths, and after it was dedicated in 25 BC, the porticus may have been the center for the public aspects of the Neptunalia. (Agrippa was apparently a great fan of Neptune, and coins honoring Agrippa routinely featured Neptune.) This Neptune complex was completely restructured by Hadrian when he also rebuilt the Pantheon, so it is difficult to discern the Agrippan design and functions of the structure.
Neptune was also held in high regard as Neptune Equestor, the god and patron of the extremely popular horse races and of horses. As early as 206 BC, one of his temples was located near the Circus Flaminius, one of the larger trace-tracks and the place where the equites, members of the equestrian class, mustered for their ceremonies outside the pomerium or ceremonial boundary of the city. The horse connection was obvious: Neptune Oceanus was often depicted surfing on a sea shell towed by "sea horses" (actually hippocampi, half horses and half fish -- in front to back order, of course: think of the alternative!) Part of the Neptune myth is that he gave the first horses to men.
The areas around the Circus Flaminius and the Pantheon sometimes flooded after spring rains in obvious manifestations of Neptune. A coin issued around 40 BC depicted the Circus Flaminius Neptune temple on a podium without an approaching staircase, and this has led to conjecture that the temple might only have been used during floods when it would be approached by boats.
Neptune usually carried the identifying trident and was accompanied by dolphins. Rome's most famous shell-surfing Neptune is not Ancient Roman at all. The twenty-foot-high marble statue was finished in 1761 by Pietro Bracci, under the direction of architect Nicola Salvi, as the centerpiece of the Trevi fountain. Fittingly, it marks the end of the Virgine aqueduct, which was originally extended to the Piazza di Trevi by that same Agrippa who built the Temple of Neptune not far away at the Pantheon.
The Neptunalia was a typical Mediterranean mid-summer festival, falling halfway between the summer solstice and the vernal equinox. Northern, and especially Keltic, midsummer usually fell on the summer solstice in mid June: the Keltic summer began on May 1. Both Mediterranean and Northern mid-summer festivals involved a return to nature, although in northern areas, where summer rain was more common, there did not appear to be any specific emphasis on celebration, prayer, and sacrifice to the water god. The Jewish "Feast of Tabernacles", Succot, although it has the building of "tabernacles" in common with the Neptunalia, is a festival that occurs at the time of the second or autumnal harvest. Orthodox Jewry says that the festival commemorates the temporary shelters that Jews lived in during the exodus from Egypt, but some secular Jewish scholars say they were simply the temporary shelters erected for overnight stays in the fields during harvest time. The likely fusion was that spending time in the fields in temporary shelters during the harvest also reminded the Jewsof their time in the wilderness.
Smith's dictionary on Roman Religion, with a Neptunalia link: http://www.ukans.edu/history/index/europe/ancient_rome/E/Roman/Texts/secondary/SMIGRA*/Religion/home*.html
Platner and Ashby on Sanctuaries of Neptune in Rome: http://www.ukans.edu/history/index/europe/ancient_rome/E/Gazetteer/Places/Europe/Italy/Lazio/Roma/Rome/.Texts/PLATOP*/Neptunus.html
Neptune/Poseidon mythology: http://ancienthistory.about.com/homework/ancienthistory/library/bl/bl_myth_gods_grecoroman_poseidon.htm
Trevi fountain -- don't drink the water! http://www.mmdtkw.org/VTrevi.html
His little brother's feast -- Saturnalia: http://www.mmdtkw.org/VSaturnalia.html
The other Neptune, from NASA: http://pds.jpl.nasa.gov/planets/welcome/neptune.htm