Glossed in other lexica as "prominent" (Etymologicum Gudianum) or "prominent or invited" (ps.-Zonaras). The latter provides the context for the word: "as in: This invited and holy day" (klhth\ kai\ a(gi/a h(me/ra). This is an approximation of the phrase with which the Septuagint describes the Sabbath, klhth\ a(gi/a (Exodus 12:16, Leviticus 23, Numbers 28:25); and outside the fourth-century BCE historian D(e)inon of Kolophon (FGrH 690 F7), it is the first attested use of the feminine nominative cited.
The confusion among lexicographers between "invited" and "exceptional" is because the Septuagint (if it is interpreted on the basis of the Hebrew מִקְרָא-קֹדֶשׁ miqra qodesh) apparently uses klhth/ not as the adjective "invited" (as ps.-Zonaras construes it, and cf. kappa 1798), but as a noun, "invitation, assembly" (LSJ s.v. klhto/s II): the Sabbath is a "holy convocation" (KJV), a day to "hold a sacred assembly" (NIV). The Hebrew מִקְרָא miqra comes from the stem קרא "call; read"; the Hebrew could mean "occasion", "to be called", as well as "assembly".
Alternatively, the Septuagint may be using klhth\ ... e)/stai as an analytic future perfect passive ("shall be called"), in parallel with the future passive klhqh/setai in the previous clause. For analytic tenses using the verbal adjective instead of a participle, see Conybeare, Grammar of Septuagint Greek, p. 70 (web address 1). If this is correct, the Septuagint meaning differs from the Hebrew, but the parallel structure is maintained: "The first day shall be called holy, and the seventh day shall be [a day] called holy by you." The pronoun u(mi=n would be dative of interest or agent.