Until the day in which (achri hēs hēmeras). Incorporation of the antecedent into the relative clause and the change of case hēi (locative) to hēs (genitive).
Was received up (anelēmpthē). First aorist passive indicative of analambanō. Common verb to lift anything up (Act 10:16) or person as Paul (Act 20:13). Several times of the Ascension of Jesus to heaven (Mar 16:19; Act 1:2, Act 1:11, Act 1:22; 1Ti 3:16) with or without “into heaven” (eis ton ouranon). This same verb is used of Elijah’s translation to heaven in the lxx (2 Kings 2:11). The same idea, though not this word, is in Luk 24:51. See note on Luk 9:51 for analēmpsis of the Ascension.
Had given commandment (enteilamenos). First aorist middle participle of entellō (from en and tellō, to accomplish), usually in the middle, old verb, to enjoin. This special commandment refers directly to what we call the commission given the apostles before Christ ascended on high (Joh 20:21-23; Mat 28:16-20; Mar 16:15-18; 1Co 15:6; Luk 24:44-49). He had given commands to them when they were first chosen and when they were sent out on the tour of Galilee, but the immediate reference is as above.
Through the Holy Spirit (dia pneumatos hagiou). In his human life Jesus was under the guidance of the Holy Spirit. This applies to the choice of the apostles (Luk 6:13) and to these special commands before the Ascension.
Whom he had chosen (hous exelexato). Aorist middle indicative, not past perfect. The same verb (eklexamenos) was used by Luke in describing the choice of the twelve by Jesus (Luk 6:13). But the aorist does not stand “for” our English pluperfect as Hackett says. That is explaining Greek by English. The Western text here adds: “And ordered to proclaim the gospel.”