What is the X in X-mas?
late 14c., the seven daughters of Atlas and Pleione, transformed by Zeus into seven stars, from Latin Pleiades, from Greek Pleiades, perhaps literally "constellation of the doves" from a shortened form of peleiades, plural of peleias "dove" (from PIE root *pel- "dark-colored, gray"). Or perhaps from plein "to sail," because the season of navigation begins with their heliacal rising.
Old English had the name from Latin as Pliade. Mentioned by Hesiod (pre-700 B.C.E.), only six now are visible to most people; on a clear night a good eye can see nine (in 1579, well before the invention of the telescope, the German astronomer Michael Moestlin (1550-1631) correctly drew 11 Pleiades stars); telescopes reveal at least 500. Hence French pleiade, used for a meeting or grouping of seven persons.
Heb. kimah, "a cluster" (Job 9:9; 38:31; Amos 5:8, A.V., "seven stars;" R.V., "Pleiades"), a name given to the cluster of stars seen in the shoulder of the constellation Taurus.