What will ye do in the solemn day - When ye shall be despoiled of every thing by the Assyrians; for the Israelites who remained in the land after its subjection to the Assyrians did worship the true God, and offer unto him the sacrifices appointed by the law, though in an imperfect and schismatic manner; and it was a great mortification to them to be deprived of their religious festivals in a land of strangers. See Calmet.
What will ye do in the solemn day? - Man is content to remain far from God, so that God do not show him, that He has withdrawn Himself from him. Man would fain have the power of drawing near to God in time of calamity, or when he himself likes. He would fain have God at his command, as it were, not be at the command of God. God cuts off this hope altogether. he singles out the great festivals, which commemorated His great doings for His people, as though they had no more share in those mercies. The more solemn the day, the more total man‘s exclusion, the more manifest God‘s withdrawal. To one shut out from His service, the days of deepest religious joy became the days of deepest sorrow. Mirth is turned into heaviness. To be deprived of the ordinary daily sacrifice was a source of continual sorrow; how much more, “in the days of” their “gladness” Numbers 10:10, in which they were bidden to rejoice before the Lord, and “in which they seemed to have a nearer and more familiar access to God.” True, that having separated themselves from the temple, they had no right to celebrate these feasts, which were to be held in the place “which God had chosen to place His name there.” Man, however, clings to the shadow of God‘s service, when he has parted with the substance. And so God foretold them before, that He would “make all their mirth to cease” Hosea 2:11.