Somewhat conjectural²

When the Elves first travelled to Valinor, they discovered a land filled with the shining Light of the Trees. As beautiful as they found the Light, however, the Elves had been born under starlit skies, and still longed to see the stars. Thus the Valar made a ravine running through the eastern mountain-border of their realm, looking out to the lands and sea beyond where the stars still shone down.

That cleft in the mountains was named the Calacirya, the Pass of Light, and the Elves raised a city within it that they named Tirion. They also dwelt in the lands about the ravine, in a region that came to be known as Eldamar or Elvenhome. That part of Eldamar that lay beyond the end of the pass of the Calacirya was known as the Calacirian, an open land filled with the Light of the Trees, and a place of astonishing beauty.

Tolkien is explicit that the place known as the Calacirian lay at the opening of the Pass of Light, he does not specifically mention which end of the ravine it occupied. Based on the available evidence, however, it seems to have been at the western end (that is, the place where the Calacirya opened onto the plains of Valinor). Notably, the Light of the Trees was said to have been at its brightest here, and the western end of the cleft would have been the nearest to the Two Trees. What's more, in Bilbo's "Song of Eärendil" in Rivendell, he describes Eärendil travelling first to Tirion, and then passing through the Calacirian into Valinor, which would only make sense if the Calacirian was at the western end of the pass of the Calacirya.

It should be said that the meaning of the name Calacirian was not always as unambiguous as these notes might imply. In textual changes from 1958 (that is, after the initial publication of The Lord of the Rings), Tolkien was still pondering possible alternative meanings. In volume X of The History of Middle-earth, there's a record of him considering changing Kalakirya (the name of the Pass of Light using an older spelling convention) to Kalakiryan. So, at least for a time, the Calacirian might have been considered as a name for the pass, or part of it, rather than an open land at its end.