Exegesis:
egeneto de lit. ‘and it happened,’ cf. on 1.8.
en tō sumplērousthai tas hēmeras tēs analēmpseōs autou ‘when the days of his ascension were fulfilled.’ For this phrase cf. on 1.57 (where a related verb is used). tas hēmeras refers to a period which must elapse before the event to which tes analēmpseōs refers, can take place. sumplēroō, cf. on 8.23.
analēmpsis lit. ‘taking up,’ here in a passive sense ‘being taken up,’ with a shift to another word picture, ‘ascension’ (cf. the related verb analambanomai in Acts 1.2, 11, 22).
kai autos to prosōpon estērisen tou poreuesthai eis Ierousalēm lit. ‘he set his face in order to travel to Jerusalem.’ The meaning of the Hebraistic phrase to prosōpon estērisen is ‘he resolved firmly,’ ‘he decided definitely.’ tou poreuesthai eis Ierousalēm is an independent genitive of the articular infinitive, loosely attached to the main clause in order to indicate its purpose.
stērizō ‘to fix,’ ‘to set,’ ‘to strengthen.’
(v. 52a) aggelous ‘messengers.’
pro prosōpou autou lit. ‘before his face,’ hence ‘before him,’ ‘ahead (of him).’
Translation:
Solemn wording and contents of v. 51 serve to make the fact that a new major part of the narrative (9.50–19.27) begins here.
When the days drew near for him to be received up, or, ‘that he/Jesus was to be received up, or, was to go up’; or focussing rather on the preceding period, ‘as the days before he should be received up were running out’ (cf. Phillips), ‘when the time that had to elapse before his ascension was almost at its end’ (Leyden). An interesting idiom for time that has elapsed is ‘knot that is cut’ (Ekari, cf. the Tboli rendering of “at the end of eight days” in 2.21). The idea that all this was preordained can usually be expressed in the phrase referring to the coming event (see above), but is sometimes better expressed (also) in the reference to the period, e.g. ‘when the time-agreed-upon was at hand’ (Balinese), ‘when the time was due to come that…’ (Sranan Tongo). To be received up. A locative qualification is often desirable, e.g. ‘that Jesus would be taken-up/made-to-go-up (or, would go up) to heaven’ (Tzeltal, Balinese, Sranan Tongo), ‘that he should be taken away from this world’ (Bible de Jérusalem). — Some translators have chosen a euphemism for ‘to die,’ e.g. ‘to go home,’ ‘to pass away,’ or a term for ‘elevation (to a higher rank)’; this is not to be recommended.
He set his face to go to, or, “he resolved to go to” (Phillips), ‘Jesus decided to take-hold-of road to go to’ (Kituba), ‘he took resolutely the road to’ (Bible de Jérusalem). Idioms used to express firm resolve are, ‘to put into the head’ (Sranan Tongo), ‘to cause-to-be-fixed the heart’ (Bahasa Indonesia RC), ‘straight before the nose’ (Ekari), ‘to hold-down the will’ (Pohnpeian, Trukese).
(V. 52a) He sent messengers ahead of him, or, to avoid tautology, ‘he sent some persons ahead of him,’ see on 7.27.