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.

Les explains that Peter was seeing Joel's prophecy being fulfilled on the Day of Pentecost, but Peter should have stopped before verse 19 if he had known Israel would go into dispersion and God would turn to John. Instead, Peter kept hanging on the Old Testament program: 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. So when Peter continues Joel's quotation about wonders in heaven, signs in the earth, blood, fire, vapor of smoke, the sun darkened, and the moon turned to blood, Les says he is talking about the tribulation. Peter knew it had to come. The passage ends with whoever calls on the name of the Lord shall be saved, which Les notes is Peter, not Paul.

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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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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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.