Albert Barnes Commentary Daniel 10:2

"In those days I, Daniel, was mourning three whole weeks." — Daniel 10:2 (ASV)

In those days I Daniel was mourning - I was afflicting myself; that is, he had set apart this time as an extraordinary fast. He was sad and troubled. He does not say why he was so troubled, but there can be little doubt that it was on account of his people. This was two years after Cyrus had given the order for the restoration of the Hebrew people to their country. It is probable that they met with many embarrassments in their efforts to return, and possibly some difficulties on the subject may have arisen in Babylon that greatly affected Daniel's mind.

The difficulties involved in such an undertaking as restoring a captured people to their country, when the journey crossed a vast desert, would at any time have made an extraordinary season of prayer and fasting appropriate.

Three full weeks - Margin, “weeks of days.” Hebrew, “Three sevens of days.” He does not say whether he had intentionally set apart that time for a season of fasting, or whether, under the influence of deep feeling, he had continued his fast from day to day until it reached that period. Either possibility is consistent with the circumstances of the case, and either would have justified such an act at any time. For it would undoubtedly be proper to designate a time of extraordinary devotion or, under the influence of deep feeling—whether from domestic trouble or national affliction—to continue such religious exercises from day to day.