What day is the “Lord’s Day” in Revelation 1:10?

I was in the Spirit on the Lord’s Day, and I heard behind me a loud voice, as of a trumpet.

Various options have been suggested for this ‘Day’ (the Sabbath/Passover or the Day of Judgment); however, the reference to a ‘trumpet’ indicates that it refers to the beginning of the New Year, which is closely associated with the Day of Atonement (10 days later), referred to by Isaiah as “an acceptable day to the Lord” (Isa 58:3-5—a ‘Lord’s day’). On ‘the Day’ the ‘books’ were opened (cf. Dan 7:10) and the fate of the wicked and the righteous determined.

The 10 days between New Year and the Day of Atonement were figuratively a time of probation and trial (you will have tribulation ten days—Rev 2:10). Significantly, Daniel expected a great Day of Atonement at the end of time (for him at the end of 490 years) when the books would be opened, and he also experienced 10 days of trial.  The ‘loud voice’ in Rev 1:10 is associated with calling the dead to life (cf. John 11:43) and therefore with the judgment and in the initial vision John experiences (figuratively) his own death and resurrection (Rev 1:17).