And in the four and twentieth day of the first month, as I was by the side of the great river, which is Hiddekel;
In the four and twentieth day of the first month - Nisan, the month most suited for considering Israel's calamity, being that in which the feast of unleavened bread reminded them of their Egyptian bondage. Daniel mourned not merely for the seven days appointed (Exodus 12:18), from the evening of the 14th to the 21st of Nisan, but thrice seven days, "three full weeks" (Daniel 10:2), to mark extraordinary sorrow. His mourning ended on the 21st day, the closing day of the Passover feast; but the vision is not until the 24th, because of the opposition of "the prince of Persia" (Daniel 10:13).
I was by the side of the great river - in waking reality, not a trance Daniel 10:7): when younger, he saw the future in images, but now, when old, he receives revelations from angels in common language - i:e., in the apocalyptic mode. In the patriarchal period God often appeared visibly - i:e., by theophany. In the prophets, next in the succession, the inward character of revelation is prominent. The consummation is when the seer looks up from earth into the unseen world, and has the future shown to him by angels - i:e., apocalypse. So in the New Testament there is a parallel progression: God in the flesh, the spiritual activity of the apostles, and the apocalypse (Auberlen).
Which is Hiddekel - the Tigris.