When Mardil adjusted the calendars of the Dúnedain in III 2060, two millennia after Isildur's time, he introduced some changes to Yavannië. Other alterations to the calendar meant that the month now started a day earlier, on modern 22 August. On this new reckoning, the so-called Stewards' Reckoning, Yavannië was still thirty days in length, but it was no longer followed directly by Narquelië. Instead, Mardil introduced a new feast day, independent of any month. This was Yáviérë, a harvest holiday that followed immediately after Yavannië and was followed in turn by Narquelië.
Notes
1
This conversion to a modern calendar is based on the later Stewards' Reckoning of Gondor. The earlier Kings' Reckoning also contained month named Yavannië, which started one day later (so by that system the month would have run from modern 23 August to 21 September).
2
The name of this month derives from Yavanna, the name of the Vala of harvest and plenty (because the month of Yavannië marked the time of harvest). Yavanna's name in turn means 'Giver of Fruits' (from yávë 'fruit' and anna 'gift'). The day known as Yáviérë that followed Yavannië in the Stewards' Reckoning was etymologically connected; that name literally meant 'fruit day', but in context would probably be interpreted 'harvest day'. Also related was Yávië, 'time of fruits' or 'time of harvest', the name used by the Elves for the early part of the season of autumn.