Thanksgiving to Eru

Beyond a reference to the end of autumn, we're not told exactly when the day of Thanksgiving fell during the year, but it seems that the Númenóreans considered Yavannië to be the last month of autumn, and the following month of Narquelië ('fading Sun') to mark the onset of winter. (When the later Dúnedain introduced a specific autumn feast day, Yáviérë, they inserted it between these two months, emphasising the importance that they gave to this time of year.)

It should be noted that there is a certain amount of apparent disagreement over the date of the Thanksgiving. In Unfinished Tales, the commentary titled A Description of the Island of Númenor seems unequivocal on the dating, speaking of the '...thanksgiving to [Eru] at the Eruhantalë at the end of autumn'. In Aldarion and Erendis, however, there's a comment that the '...summer was nearly over and the Eruhantalë was nigh.', which seems to imply that the Thanksgiving to Eru came at the beginning of autumn, rather than its end.