What is your understanding of the prophecy from Joel, that according to Peter began to be fulfilled when the Spirit was poured out on all flesh, meaning not only to Jews, but Gentile believers also?
Writing of the day of the Lord that is coming and near at hand, the prophet Joel says in those day(s) when the Spirit is poured out upon all flesh, and Peter quoting the prophet writes of this time as "the last days" when all that is written of the Messiah to come would be fulfilled.
Joel 2:1-2 (KJV) Blow ye the trumpet in Zion, and sound an alarm in my holy mountain: let all the inhabitants of the land tremble: for the day of the LORD cometh, for it is nigh at hand; A day of darkness and of gloominess, a day of clouds and of thick darkness, as the morning spread upon the mountains: a great people and a strong; there hath not been ever the like, neither shall be any more after it, even to the years of many generations.
Joel 2:27-32 (KJV) And ye shall know that I am in the midst of Israel, and that I am the LORD your God, and none else: and my people shall never be ashamed. 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.
Acts 2:14-21 (KJV) But Peter, standing up with the eleven, lifted up his voice, and said unto them, Ye men of Judaea, and all ye that dwell at Jerusalem, be this known unto you, and hearken to my words: For these are not drunken, as ye suppose, seeing it is but the third hour of the day. But this is that which was spoken by the prophet Joel; 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: And it shall come to pass, that whosoever shall call on the name of the Lord shall be saved.
We know from what is written in the New Testament that the last days began to be fulfilled with the first advent of Christ. That was the beginning of the gospel of Jesus Christ, the day of the Lord the prophets foretell. These last days mark the beginning of the gospel of Jesus Christ, as it is written in the prophets, and would be known through John the Baptist, the messenger of the Lord ordained to be the voice crying in the wilderness the day of the Lord has come in these last days, telling the people to repent for the One the prophets promised would come as the day of the Lord had finally come with salvation to all who repent and believe.
Hebrews 1:1-3 (KJV) God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets, Hath in these last days spoken unto us by his Son, whom he hath appointed heir of all things, by whom also he made the worlds; Who being the brightness of his glory, and the express image of his person, and upholding all things by the word of his power, when he had by himself purged our sins, sat down on the right hand of the Majesty on high;
Mark 1:1-2 (KJV) The beginning of the gospel of Jesus Christ, the Son of God; As it is written in the prophets, Behold, I send my messenger before thy face, which shall prepare thy way before thee.
Isaiah 40:3 (KJV) The voice of him that crieth in the wilderness, Prepare ye the way of the LORD, make straight in the desert a highway for our God.
The only time that shall be left for this earth when the seventh/last trumpet begins to sound is Satan's little season. How can that be the day of the Lord since it is clearly a very small amount of time given to Satan that will end in him being utterly destroyed? The prophets speak of the day of the Lord as the time when man can call upon the name of the Lord to be saved! That cannot be a short amount of time and certainly not one thousand literal years. It will take much time for the day of the Lord to accomplish the mission of building the spiritual Kingdom of God in heaven as the gospel of the Kingdom of God is being proclaimed unto all the nations of the world.