Acts 2: the derivation of 'Pentecost'

While I continue to work on speed-learning HTML code, we'll start Acts 2.  Recall that at the end of Acts 1, Matthias has just been chosen to replace Judas.  In Acts 2:1, the disciples are gathered together on the day of Pentecost.

Καὶ ἐν τῷ συμπληροῦσθαι . . . . And in the to fulfill  

τὴν ἡμέραν τῆς Πεντηκοστῆς, . . . . the day of Pentecost

ἦσαν ἅπαντες . . . . they were all

ὁμοῦ ἐπὶ τὸ αὐτό. . . . together on the place

And when the day of Pentecost came, they were all together in one place.

The form συμπληροῦσθαι is the present middle/passive infinitive of συμπληρόω - 'I fill, fill completely, fulfill'.

Ἠ Πεντηκοστῆ is translated as 'Pentecost', but is more literally 'fiftieth', with 'day' understood from context.  This word derives from πέντε, 'five', and the ending -κοστος, both known from classical times down, with -κοστος declined as usual for an adjective.  

This ending corresponds to the '-ieth' or '-eth' endings of English, e.g., twentieth, thirtieth, two-hundredth and so on.  Thus we see the form πεντηκοστῆς used here, since 'day' (ἡμέρα) is feminine, and the genitive case is by sense.

The adverb ὁμοῦ ('together, at the same place / time') is replaced in the Byzantine Majority Text by the adverb ὁμοθυμαδὸν, which is generally translated with something like 'with one accord'.  Ὁμοθυμαδὸν is actually used more often in the New Testament than ὁμοῦ.