Although the word “day” can refer to just an ordinary, unspecified time, it is often used in the prophetic books as a short form of “the Day of the Lord” (Jeremiah 46:10, Jeremiah 47:4, Ezekiel 7:10–19, Ezekiel 30:3), the day of judgment when God executes justice, rewarding the righteous, but punishing the wicked. It is clear from Malachi 4:1 that this is the way that “the day” is used in this verse.
There remains the question as to whether this “day” points to the ultimate Day of Judgment when Christ returns, or to some prior judgment, such as the destruction of Jerusalem in AD 70.
17 “They shall be mine, says the LORD of hosts, in the day when I make up my treasured possession, and I will spare them as a man spares his son who serves him.