After the Angel (High Priest) had cast the Golden Censor to the earth (Revelation 8:5), John noticed that the seven angels with the seven trumpets were preparing themselves to sound out (Revelation 8:6). It seems awfully unnecessary to say “the seven angels… prepared themselves to sound,” if all they had to do was draw a breath. No preparations were necessary for the Four Horsemen in Revelation 6:1-8, nor were any preparations necessary for the first four angels who poured out their vials of judgment upon Babylon in Revelation 16:1-9. Yet, concerning the angels who would blow the trumpets, it was said they “…prepared themselves to sound” (Revelation 8:6). What can we say of this?
In ancient Israel trumpets were used for various reasons. For example, they were used to call the people to assemble before the Lord to worship (Numbers 10:3-4, 7, 10; Psalm 81:3-4). They were also used to introduce a joyous occasion, such as the beginning and the consummation of the building of the Temple and its walls (Ezra 3:10-11; Nehemiah 12:35, 41), for the year of jubilee (Leviticus 25:9), and for the coronation of the king (1Kings 1:34, 39). Trumpets were also used to sound the alarm, meaning to alert the people of coming trouble, so they could take the proper precautions, like fleeing to a defensible city (Numbers 10:9; Ezekiel 33:3-6; Joel 2:1). Finally, they were also used to gather the people for war against their enemies (Numbers 31:6; Judges 3:27; 6:34; 1Samuel 13:3)
The first four angels are identified as the Four Winds of Revelation 7:1-3 who hold back their service, until the disciples of Jesus are sealed in their foreheads. They stood at the four corners of the earth (Revelation 7:1), meaning the borders of the land of the Jews. In Revelation 8:2 they are shown to be standing before the Throne, which is where the Golden Altar is. This is the same altar, at which the Angel with the golden censor stood offering the prayers of the saints (Revelation 8:3-5). In other words, the angels, who were holding back the winds at the four corners of the land of the Jews, were standing near the four corners of the Golden Altar (Exodus 37:25-26). The four horns, also overlaid with gold, were fixed upon the four corners of the altar. The Golden Altar, or the Altar of Incense, represented the prayers coming from the people of God within the four corners or the borders of the land of his people (cf. Revelation 8:3), and its four horns seem to indicate the power of prayer. Hence, it seems that the judgments heralded by these angels were delayed until the prayers of the saints, viz. at the opening of the fifth seal (Revelation 6:9-11), could be answered.
Just as the first four seals, identified as the Four Horsemen of the Apocalypse (Revelation 6:1-8), were separated from the last three seals, so, too, are the first four trumpets, which we can identify as the Four Winds (Revelation 7:1), separate from the last three trumpets, which themselves are identified as the Three Woes (Revelation 8:13).
If it is logical and accurate to say that the first four angels are the same angels who held back the four winds, until the elect of God were sealed (Revelation 7:1-3), then it may be significant to understand what constitutes the sealing of the elect, and how long it might have taken for the sealing to occur. If we can say that the actual sealing process points to the receiving of the Holy Spirit, then Pentecost of Acts 2 is the sealing of at least some of the elect. If we are able to accept that the Cornelius issue of Acts 10 was a similar event, just as Peter, himself, concluded in Acts 10:44-48 and in Acts 11:15-17, then we could say that a sealing of the elect occurred representing the Jews in Acts 2 and the gentiles in Acts 10. This represents a period of about 8 years (31 AD to 39 AD).
Nevertheless, this isn’t all we need to consider. Yet, another sealing occurred as Paul spoke to new believers in Ephesus (Acts 19:1-7). The twelve men there represented the 10 lost tribes of Israel, or the House of Israel that went into captivity before the House of Judah (cir. 54 AD). Therefore, the anointing or sealing of the whole of the elect with the Holy Spirit was begun on these three separate occasions. The ministries of Peter, Paul and James ended sometime in the 60s AD with their deaths, James, the brother of the Lord, by stoning cir. 62 AD, and both Peter and Paul at the hand of Nero, probably 64-66 AD. It seems evident to me that these three events, Acts 2, 10 and 19, are very significant in that they represent what occurred in Revelation 7:1-8, and this represents the preparations that had to be made before the first four angels could blow their trumpets. Once the sealing of the twelve men in Ephesus was complete (cir. 54 AD), or perhaps when Paul was imprisoned in 56 AD, it seems that the sealing work (Revelation 7:1-8) was done. So, whatever the seven trumpets mean, they were free to be blown and their judgments executed from about 54 AD (or 56 AD) forward, until the seventh trumpet was blown in Revelation 11:15-19 (cir. 66 AD), roughly a ten to twelve year period.
 Moses never said from what beast the horns came that decorated the four corners of the Altar of incense. However, Psalm 22:21 implies they were the horns of the unicorn, which seems to be that of a massive wild ox, not a single horned creature such as the rhinoceros, because out of Joseph came two tribes, Ephraim and Manasseh (cf. Deuteronomy 33:17). This beast was ferocious (Numbers 23:22; 24:8) and untamable (Job 39:9-12). Such an animal would be extinct today.
 See Josephus: Antiquities of the Jews 20.9.1.