The religious festivals such as the Passover had been given to Israel as both a privilege and an obligation. Observing these festivals was how Israel expressed her commitment to the Lord. To ignore their vows and festivals is to ignore the Lord.1 The good news of being set free from oppression under Assyria would release Judah to live out their covenant relationship with the Lord once again by keeping her feasts and fulfilling her vows. This is true even while under Assyrian oppression, because the good news has come—that the Lord is bringing peace to Judah. He will utterly cut off their enemy forever—ending oppression and restoring true worship amongst his people.
This is always the pattern of God’s salvation. God acts to save his people by destroying the enemy and setting his people free; even though his people do not deserve this. What flows from this undeserved salvation is a life of commitment to the Lord.
15 Behold, upon the mountains, the feet of him who brings good news, who publishes peace! Keep your feasts, O Judah; fulfill your vows, for never again shall the worthless pass through you; he is utterly cut off.