the day. Put by Figure of speech Metonymy (of Subject), for the events (or judgments) which shall take place in it.
day of the Lord.. See note on Isaiah 2:12. Occurs in fifteen other places in O.T.: (Isaiah 13:9; Ezekiel 13:5.Joel 1:15; Joel 2:1; Joel 2:11; Joel 2:31; Amos 5:18; Amos 5:18; Amos 5:20; Obadiah 1:15.Zephaniah 1:7; Zephaniah 1:14; Zephaniah 1:14.Malachi 4:5 (total 4x4, App-10).
destruction... ALMIGHTY. Note Figure of speech Paronomasia. Hebrew. keshod... mishshaddai.
the ALMIGHTY. the All-bountiful One. Hebrew. Shaddai (App-4).