Neptune's festival Neptunalia is celebrated on July 23rd and 24th. In the middle of ancient Italy's really hot and dry summer season, when rivers were low and water was most scarce, Romans sought Neptune's protection of irrigation waters and water works (aqueducts, wells, etc.)
Virtually nothing is known about how the two day festival was celebrated, except that the people used to build huts of branches and foliage in which they probably feasted, drank, and amused themselves. A bull might be sacrificed to Neptune and cooked over open fires and the revelers would typically spend the night under the stars and celebrate on into the next day.
Honoring Neptune on these days would assure rainfall for the crops and prevent a drought. Neptunalia was also considered to be an auspicious day to start new irrigation works, which were easier to construct in summer than during other wetter seasons.
Some places have revived this festival! The citizens of Fremont celebrate it with a parade and other festivities. How will you celebrate Neptunalia?