The modern calendar of Easter Island has two autumn months Vaitu nui (April) and Vaitu potu (May). Equally, there are two early spring months with similar names: Hora iti (August) and Hora nui (September). Probably there once were only 10 months and the double names were created by splitting up the old months Vaitu and Hora.

The sense in Vaitu, I imagine, could be 'water' (vai) at the backside (tu'a). The old name may have been Vaitu'a, '(the) water at the backside (of the) year'. The front (ra'e) side would then be the Hora side, the time when sun is advancing upwards.

The sea turtle naturally will be associated with the watery season. Its resistance to fire is an additional factor which puts the turtle in opposition to the sun.

(Green Sea Turtle, ref. Wikipedia)