From Tolkien Gateway

Firith was the Sindarin name for the season of late autumn, and called Quellë in Quenya. In the Reckoning of Rivendell it was fourth of the six "seasons", and corresponded more or less to the 54 days lying between 7 October and 30 November in the Shire Calendar.[1]

See also: Narbeleth


Calendar of Imladris
'First Day' 'Spring' 'Summer' 'Autumn' 'Middle Days' 'Fading' 'Winter' 'Stirring' 'Last Day'
Quenya: Yestarë Tuilë Lairë Yávië Enderi Quellë Hrívë Coirë Mettarë
Sindarin: Ethuil Laer Iavas Firith Rhîw Echuir