One day. A unique day, unparalleled (comp. So Zechariah 6:9; Ezekiel 7:5). Which shall be (is) known to the Lord. Its peculiar character, and the moment of its arrival, are known to God, and God only (Matthew 24:36).
Not day, nor night. It cannot be called truly the one or the other, because there is darkness in the day and light at night, as the following clause says. This is symbolically explained by St. Ephraem, "It will not be altogether consolation, nor altogether affliction."
It is not full daylight, for calamity presses; it is not deep night, because there is hope amid the distress. At evening time it shall be light. In the midst of trouble and danger deliverance shall come.
The whole section is a figurative description of the fortunes of the Church militant, even as Christ announced to his disciples: "In the world ye shall have tribulation: but be of good cheer, I have overcome the world" (John 16:33); "If they have persecuted me, they will also persecute you" (John 15:20); "Let not your heart be troubled, neither let it be afraid' (John 14:27).