Jamieson, Fausset and Brown on 1 Corinthians 15:52

52. the last trump--at the sounding of the trumpet on the last day [Vatablus] (Mt 24:31; 1Th 4:16). Or the Spirit by Paul hints that the other trumpets mentioned subsequently in the Apocalypse shall precede, and that this shall be the last of all (compare Is 27:13; Zec 9:14). As the law was given with the sound of a trumpet, so the final judgment according to it (He 12:19; compare Ex 19:16). As the Lord ascended "with the sound of a trumpet" (Psa 47:5), so He shall descend (Re 11:15). The trumpet was sounded to convoke the people on solemn feasts, especially on the first day of the seventh month (the type of the completion of time; seven being the number for perfection; on the tenth of the same month was the atonement, and on the fifteenth the feast of tabernacles, commemorative of completed salvation out of the spiritual Egypt, compare Zec 14:18, 19); compare Psa 50:1-7. Compare His calling forth of Lazarus from the grave "with a loud voice," Joh 11:43, with Joh 5:25, 28.

and--immediately, in consequence.