The Elves divided each year into periods that corresponded with the seasons, though they used six of these instead of the more familiar four. Two of these periods corresponded to our autumn: Iavas and Firith, each being fifty-four days long, divided from each other by three days known as the Enderi. Firith thus marked the darkening and waning of the year as it passed into winter, and this is reflected in the name (firith means 'fading', and the season was also called Narbeleth or 'Sun-waning').
Firith was the Sindarin name for late autumn; in Quenya the same season was named Quellë, having the same meaning of 'fading'. On a modern calendar this period would cover most of October and November; to be precise, Firith ran between 28 September and 20 November.