The name is Welsh, often translated as " The Light of the Water " or " The Zenith of Autumn ".

While the show uses it as a fantasy backdrop, is a real term used in modern Druidry to mark the Autumn Equinox , typically occurring around September 21st or 22nd.

In both the show and real-world practice, the day is defined by specific customs:

Autumn Equinox - Alban Elfed | Order Of Bards, Ovates & Druids

It is celebrated as the second of three harvest festivals (following Lughnasadh and preceding Samhain). It specifically honors the gathering of fruits like apples and nuts that ripened under the summer sun. Traditions and Rituals

It represents the moment when day and night are in perfect balance (12 hours each) before the "dark half" of the year begins.