John Gill Commentary Acts 2:20

"The sun shall be turned into darkness, And the moon into blood, Before the day of the Lord come, That great and notable [day]." — Acts 2:20 (ASV)

The sun shall be turned into darkness
As at the death of Christ, by a total eclipse of it:

and the moon into blood ;
as at the opening of the sixth seal, (Revelation 6:12)

before that great and notable day of the Lord come ;
when he shall come in power and great glory, as he did in a few years after this, to take vengeance on the Jews, and destroy their nation, city, and temple; in which there was a display of his greatness, and power, and which was awful and "terrible" to them, as in Joel it is called; (See Gill on Matthew 24:29).