The speaker is either the prophet as he speaks directly or God as mediated through the prophet. Here it appears that the prophet is giving a command based on what God has revealed to him. Thus it is the prophet, not the Lord, who is the speaker, introducing the book’s central theme: the day of the Lord.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.