It's a II* (IV A) tone antiphon used for the Solemnity of the Assumption of the Blessed Virgin Mary (Aug. 15): as offertory (Graduale Simplex, p. 299) and as second antiphon of First Vespers (Liber Responsorialis, p. 374), according to the Ordo Cantus Officii. In the new Antiphonale Romanum by the way, the second antiphon of First Vespers is Paradisi porta per Evam.
To participate in the discussions on Catholic church music, sign in or register as a forum member, The forum is a project of the Church Music Association of America.