Rapa Nui calendar
The Rapa Nui calendar was the indigenous lunisolar calendar of Easter Island. It is now obsolete. Attestation. William J. Thomson, paymaster on the USS "Mohican", spent twelve days on Easter Island from December 19 to 30, 1886. Among the data Thomson collected were the names of the nights of the lunar month and of the months of the year: Months. Thomson recorded the months as follows: Days. The month was divided in two, beginning with the new and full moon. Thomson recorded the calendar at the time of his visit to the island as follows. The new  moon occurred on November 25 and again on the night of December 24; Thompson records the crescent was first visible on November 26. The three sources we have correspond with each other except for two intercalary days (in bold), and the night of the new moon in Englert, which seems to have been confused with one of these. Beginning with "(o)ata," the night of the new moon, they are: The "kokore" are unnamed (though numbered) nights; "tahi, rua, toru, haa, rima, ono" are the numerals 1–6. The word is cognate with Hawaiian "‘a‘ole" "no" and Maori "kahore" "no" and Tahitian  ‘aore "there is/are not"; here it may mean "without [a name], nameless". Analysis. The calendar collected by Thomson is notable in that it contains thirteen months. All other authors mention only twelve, and Métraux and Barthel find fault with Thomson: However, Guy calculated the dates of the new moon for years 1885 to 1887 and showed that Thomson's list fit the phases of the moon for 1886. He concluded that the ancient Rapanui used a lunisolar calendar with "kotuti" its embolismic month ( "leap month"), and that Thomson chanced to land on Easter Island in a year with a  leap month. The days "hotu" and "hiro" appear to be intercalary. A 28-day calendar month needs one to two intercalary days to keep in phase with the 29½-day lunar month. One of the rongorongo tablets may describe a rule for when to add these days.