Commentaries:
This actually confirms that God permits national observances. His complaint is not with the observance of the fasts per se, but with the attitude in which the Jews observed them. The Jews' attitude abused something permitted but not commanded. God expresses His disapproval of the ethical and spiritual attitudes that underlay their outward observance. He questions their sincerity and motivation during their fasts, which should have been times of prayer and repentance. They should have used the time to recall the sins that had led them into the slavery that made calling the fasts so necessary. They should have been searching for any remnant of those sins still residing in them and repenting of them. In Isaiah 58:5, God asks, "Is it a fast that I have chosen?" God is scolding the Jews in the same way.
John W. Ritenbaugh
Thanksgiving or Self-Indulgence?