From Tolkien Gateway

Yavannië (Ivanneth in Sindarin) was the ninth month of the year in the King's Reckoning and the Stewards' Reckoning of Gondor, following Úrimë, and preceding the month of Narquelië. In the Stewards' Reckoning, Yavannië preceded the autumn feast of Yáviérë. In the New Reckoning, Yavannië was the sixth month of the year (still following Úrimë) and preceded the New Reckoning's Enderi.[1]

Yavannië in the Kings' Reckoning (and for the most part in the Stewards' Reckoning) corresponded to Halimath in the Shire Calendar[1] (approximately modern 23 August through 21 September). In the New Reckoning, 30 Yavannië corresponded with the birthday of Frodo Baggins, 22 Halimath in the Shire Calendar, and was made a festival.[1]

See also: Cormarë