Was now come (en tōi sunplērousthai). Luke’s favourite idiom of en with the articular present infinitive passive and the accusative of general reference, “in the being fulfilled completely (perfective use of suṅ) as to the day of Pentecost.” Common verb, but only in Luke in N.T. In literal sense of filling a boat in Luk 8:23, about days in Luk 9:51 as here. Whether the disciples expected the coming of the Holy Spirit on this day we do not know. Blass holds that the present tense shows that the day had not yet come. It is a Hebrew idiom (Exo 7:25) and Luke may mean that the day of Pentecost was not yet over, was still going on, though Hackett takes it for the interval (fifty days) between Passover and Pentecost. Apparently this day of Pentecost fell on the Jewish Sabbath (our Saturday). It was the feast of first fruits.
All together in one place (pantes homou epi to auto). All together in the same place. Note homou here (correct text), not homothumadon as in Act 1:14, and so a bit of tautology.