John Calvin Commentary Numbers 20:29

"And when all the congregation saw that Aaron was dead, they wept for Aaron thirty days, even all the house of Israel." — Numbers 20:29 (ASV)

And when all the congregation saw. This has been an error common to almost all nations and ages, but which reigned particularly among the people of Israel—to pay due honor to God’s holy servants more after their deaths than during their lives. They had frequently wished to stone Aaron; they had raised great tumults to cast him down from the dignity in which God had placed him. Now, forgetting their malignity and envy, they mourn for him now that he is dead.

The question, however, arises whether the mourning for a month, which is recorded here, was praiseworthy or not. But it could only be considered improper, since it was a means of aggravating their grief. For men are naturally far too inclined to excessive grief, even if they do not indulge it; and besides, the hope of a better life helps to mitigate sorrow. Therefore, we infer that those who give way to immoderate grief possess scarcely any taste of eternal salvation.

However, since believers have another reason for mourning—that is, to exercise themselves both in the fear of God and in the hope and desire of the future resurrection—this solemn mourning has, not without reason, been accepted as a general custom. Since death is a mirror of God’s curse upon the whole human race, it is beneficial for us, whenever any of our own die, to mourn our common lot, so as to humble ourselves under God’s hand.

Besides, if mourning is directed to its proper end, it, in a manner, unites the living with the dead, so that in death itself the communion of the new and immortal life shines forth. Furthermore, the weakness of the ancient people needed to be propped up and supported by such aids as this. For, in the midst of their dark shadows, it would not have been easy for them to rise above the world unless they had been taught that the dead still belonged to them and that some bond of connection remained between them.

But if the usefulness (of this custom) is corrupted by its abuse, it is not just that what is right in itself should be blamed for the fault of men.