Shall be turned (metastraphēsetai). Second future passive of metastrephō, common verb, but only three times in the N.T. (Act 2:20 from Joel; Jam 4:9; Gal 1:7). These are the “wonders” or portents of Act 2:19. It is worth noting that Peter interprets these “portents” as fulfilled on the Day of Pentecost, though no such change of the sun into darkness or of the moon into blood is recorded. Clearly Peter does not interpret the symbolism of Joel in literal terms. This method of Peter may be of some service in the Book of Revelation where so many apocalyptic symbols occur as well as in the great Eschatological Discourse of Jesus in Matthew 24, 25. In Mat 24:6, Mat 24:29 Jesus had spoken of wars on earth and wonders in heaven.
Before the day of the Lord come, that great and notable day (prin elthein hēmeran kuriou tēn megalēn kai epiphanē). The use of prin with the infinitive and the accusative of general reference is a regular Greek idiom. The use of the adjectives with the article is also good Greek, though the article is not here repeated as in Act 1:25. The Day of the Lord is a definite conception without the article.
Notable (epiphanē) is the same root as epiphany (epiphaneia) used of the Second Coming of Christ (2Th 2:8; 1Ti 6:14; 2Ti 4:1; Tit 2:13). It translates here the Hebrew word for “terrible.” In the Epistles the Day of the Lord is applied (Knowling) to the Coming of Christ for judgment (1Th 5:2; 1Co 1:8; 2Co 1:14; Phi 1:10).