Day of the Lord - Quick & Clear Bible Study

What and when is the "Day of the Lord"?

Verses used in this Bible study:
Joel 1:15,16
Joel 2:11-14
Joel 2:28-31
Joel 3:1,2
Amos 5:18-20
Zephaniah 1:14-18
Isaiah 13:6-13
1 Thessalonians 5:2-4
2 Peter 3:10-13