"...through the Calacirian to hidden land forlorn he went."
From Bilbo's Song of Eärendil The Fellowship of the Ring II 2 Many Meetings
The region leading into the pass through the mountains of the Pelóri that led from the shores of the Great Sea westward towards Valinor. Its name derives from the Elvish for 'Pass of Light', a reference to the light of the Two Trees that shone through the pass in the days before the Sun and Moon.
Notes
1
The Annals of Aman in volume X of The History of Middle-earth gives an exact date for the making of the pass of the Calacirya: Valian Year 1133, or about 3,517 years before the first rising of the Sun. Presumably the region known as the Calacirian came into being, or at least gained its name, at the same date.
2
We have no map from any phase of Tolkien's writing showing the location of the Calacirian, and indeed all we have as a basis for its geography is brief note in the expanded index to The Lord of the Rings. That note places it in Eldamar near the pass of the Calacirya, so the location of the Calacirian must have been at least close to the point shown on the map for this entry.