What is "neptunalia"

The Neptunalia was an obscure archaic two-day festival in honour of Neptune as god of waters, celebrated at Rome in the heat and drought of summer, probably July 23 ( Varro, De lingua Latina vi.19). It was one of the dies comitiales, when committees of citizens could vote on civil or criminal matters. In the ancient calendar this day is marked as Nept. ludi et feriae, or Nept. ludi, from which Leonhard Schmitz (in Smith, see link) concluded that the festival was celebrated with games ( ludi). Respecting the ceremonies of this festival nothing is known, except that the people used to build huts of branches and foliage (umbrae, according to Festus, under " Umbrae"), in which they probably feasted, drank, and amused themselves ( Horace Carmina iii.28.1, &c.; Tertullian De Spectaculis ("On Celebrations") 6). Compare to the "booths" under which Jews celebrate Sukkot.