John Gill Commentary 2 Samuel 20:4

"Then said the king to Amasa, Call me the men of Judah together within three days, and be thou here present." — 2 Samuel 20:4 (ASV)

Then said the king to Amasa
Whom he had promised to make general of his army, (2 Samuel 19:13) ; and by the following order declared him such:

assemble me the men of Judah within three days ;
which was done by the sound of the trumpet, or by the proclamation of a herald; it seems that the men of Judah, who attended David to Jerusalem, were gone to their respective cities and places of abode, or there would have been no occasion for such a summons; though it is strange they should, when the men of Israel appeared so inclinable to a new rebellion:

and be thou here present ;
to take the command of them.