saw heaven opened
The vision is of the departure from heaven of Christ and the saints and angels preparatory to the catastrophe in which Gentile world-power, headed up in the Beast, is smitten by the "stone cut out without hands". (Daniel 2:34); (Daniel 2:35).
white horse
Contrast (Revelation 6:2); (Psalms 45:4).
Contrast (Matthew 21:2-5).
Day of the Lord
(Day of Jehovah). (Isaiah 2:10-22); (Isaiah 4:1-6); (Isaiah 11:10-13); (Isaiah 13:9-16); (Isaiah 24:21-23); (Isaiah 26:20-21); (Isaiah 63:1-6); (Isaiah 66:15-24); (Revelation 19:11-21).