From the figure mentioned in the text, we can see that the Sun reaches the zenith of a particular location, at local noon for twice a year.
From this we can conclude that the festivals were held twice in a year.
As Tahiti is at the latitude of, the Sun is at its zenith in mid-November and late January. So the festivals were held on November 13th and January30 th, every year.
As Oahu is at the latitude of, the Sun is at its zenith in late May and mid-July. So the festivals were held on May 28th and July 17 th, every year.