John Gill Commentary 2 Samuel 18:20

"And Joab said unto him, Thou shalt not be the bearer of tidings this day, but thou shalt bear tidings another day; but this day thou shalt bear no tidings, because the king`s son is dead." — 2 Samuel 18:20 (ASV)

And Joab said to him, you shall not bear tidings this day ,
&c.] Because Joab knew the tidings of Absalom's death would not be acceptable to the king; and Ahimaaz being a good man, and the son of a priest, for whom Joab had a respect, he would not send the tidings by him, which he was sensible would not recommend him to the king:

but you shall bear tidings another day ;
when any salvation is wrought, or victory obtained, the tidings of which will be welcome:

but this day you shall bear no tidings, because the king's son is
dead ;
and you must carry the news concerning his death, which it is not proper you should, being a priest, nor will it be to your advantage.