It is necessary to notice that the Holy Spirit has John use the plural word “days” to describe when the trumpet call from the seventh angel would be sounded. The plural can alert us to the fact that what is to happen with the sounding of the seventh trumpet (for details see Revelation 11:15–19) would require more than one day to unfold. The plural can also remind us that the seventh angel (and indeed all the trumpet blowers) do not blow their trumpets once only but do so multiple times in the course of history.
7 but that in the days of the trumpet call to be sounded by the seventh angel, the mystery of God would be fulfilled, just as he announced to his servants the prophets.