If the Palestinian fisherman was actually quoting Joel 2:28, he could be expected to be quoting the Hebrew text, which come down to us (with unknown amendments, if any) in the Masoretic text. Perhaps Peter knew Greek, and we know that Acts 2:8 says he could now speak in every language, but we would still expect him not to quote the Septuagint (LXX), both because, as a Palestinian, he could not have known it and because his Judean audience could not have known it. If his speech comes from the LXX, then it was written by the author of Acts of the Apostles, with any changes from Joel being made by that author to suit the theology of Acts.
Parallel translations:
Joel 2:28-32 (Masoretic):
"And it shall come to pass afterward, [that] I will pour out my spirit
upon all flesh; and your sons and your daughters shall prophesy, your
old men shall dream dreams, your young men shall see visions: And also
upon the servants and upon the handmaids in those days will I pour out
my spirit. And I will shew wonders in the heavens and in the earth,
blood, and fire, and pillars of smoke. The sun shall be turned into
darkness, and the moon into blood, before the great and the terrible
day of the LORD come. And it shall come to pass, [that] whosoever
shall call on the name of the LORD shall be delivered: for in mount
Zion and in Jerusalem shall be deliverance, as the LORD hath said, and
in the remnant whom the LORD shall call."
Joel 2:28-32 (LXX)
And it shall come to pass afterward, that I will pour out of my Spirit
upon all flesh; and your sons and your daughters shall prophesy, and
your old men shall dream dreams, and your young men shall see visions.
And on my servants and on my handmaids in those days will I pour out
of my Spirit. And I will shew wonders in heaven, and upon the earth,
blood, and fire, and vapour of smoke. The sun shall be turned into
darkness, and the moon into blood, before the great and glorious day
of the Lord come. And it shall come to pass that whosoever shall call
on the name of the Lord shall be saved: for in mount Sion and in
Jerusalem shall the saved one be as the Lord has said, and they that
have glad tidings preached to them, whom the Lord has called.
Acts 2:17-20
And it shall come to pass in the last days, saith God, I will pour out
of my Spirit upon all flesh: and your sons and your daughters shall
prophesy, and your young men shall see visions, and your old men shall
dream dreams: And on my servants and on my handmaidens I will pour out
in those days of my Spirit; and they shall prophesy: And I will shew
wonders in heaven above, and signs in the earth beneath; blood, and
fire, and vapour of smoke: The sun shall be turned into darkness, and
the moon into blood, before that great and notable day of the Lord
come: 21 And it shall come to pass, [that] whosoever shall call on the
name of the Lord shall be saved.
One single departure from the original is understandable in an impromptu speech, but Steven E. Runge says in 'Joel 2:28-32a in Acts 2:17-21
The Discourse and Text-Critical Implications of Variation from the LXX' the number and nature of the departures from the LXX reading have led many to conclude that more is going on here than simple quotation. He discusses:
the change from μετὰ ταῦτα to ἐν ταῖς ἐσχάταις ἡμέραις in Acts 2:17a;
the insertion of the prophetic formula λέγει ὁ θεός in Acts 2:17a;
the insertion of γε in Acts 2:18, and its impact on the information structure;
the insertion of ἄνω, σημεῖα and κάτω in Acts 2:20.
He says that that (Joel 2:28) μετὰ ταῦτα simply looks forward and declares that the events in question will happen at some time in the future. (Acts 2:17) Ἐν ταῖς ἐσχάταις ἡμέραις points to the last act of history and claims that they are part of God’s final act of redemption. The insertion of a more detailed temporal expression has the effect of recasting the promise of the outpouring of the Spirit from some point in time after the
events of Joel 2, to an eschatological time, one which Peter is announcing the arrival of in Acts chapter 2.
