What is Joel talking about with the wonders in heaven and signs on earth?

The signs and wonders Joel describes point to the tribulation. Blood, fire, vapor of smoke, a darkened sun, and a moon turned to blood occur before the great and notable day of the Lord. Peter quoted this prophecy on Pentecost, but its application belongs to that coming tribulation period.

Les explains that Peter, preaching on the Day of Pentecost, quoted the prophet Joel from the Old Testament. Joel spoke of God pouring out His Spirit on all flesh, sons and daughters prophesying, young men seeing visions, old men dreaming dreams, and servants and handmaids prophesying. Peter saw that happening at Pentecost. But Joel also spoke of wonders in heaven above and signs in the earth beneath: blood, fire, vapor of smoke, the sun turned into darkness, and the moon into blood, before the great and notable day of the Lord. Les says the question is, what is he talking about? The tribulation. Peter knew it had to come. Peter should have stopped right there before verse 19 had he known Israel was going into dispersion, but he kept holding to the Old Testament program that Israel had to come to a knowledge of their Messiah, then the tribulation could unroll, and then they would have the King and the Kingdom. Joel’s final note is that whosoever shall call on the name of the Lord shall be saved. Les points out that this is Peter, not Paul.

Joel 2:30

And I will shew wonders in the heavens and in the earth, blood, and fire, and pillars of smoke.

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Acts 2:19

And I will shew wonders in heaven above, and signs in the earth beneath; blood, and fire, and vapour of smoke:

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What is Joel referring to when he speaks of wonders in heaven, blood, fire, and the sun darkened before the day of the Lord?

Joel's wonders in heaven and signs on earth—blood, fire, vapor of smoke, the sun turned to darkness, and the moon to blood—refer to the tribulation. They come before the great and notable day of the Lord. Peter kept quoting Joel because he still expected Israel's Old Testament program to unfold.