It is not certain where Zephaniah derives his concept of the day of the Lord. The term first appears in Amos. From his prophecy it seems that the northern kingdom of Israel believed that the day would consist of God’s judging their enemies. Amos confirms that the day of judgment is indeed coming, but he counters the optimism of Israel by declaring that for God’s people it will be a day of disaster, not deliverance (Amos 5:18).
The prophet Isaiah also understood the day of the Lord to be a day of divine judgment against the proud among the people of Jacob (Isaiah 2:6–22).
It is possible to trace Zephaniah’s concept of the day of the Lord back to Amos. Yet while there are some similarities between Amos’s and Zephaniah’s concept of the day, Zephaniah has a greater tendency to allude to texts from Isaiah. Thus, it is more likely that the base of Zephaniah's message concerning the day of the Lord comes from Isaiah, with some influence from Amos.1
7 Be silent before the Lord GOD! For the day of the LORD is near; the LORD has prepared a sacrifice and consecrated his guests.