As in the previous verse, we again find in this verse a verb that can be understood either as a transitive verb (taking an object, which in this case would be translated as in the ESV “make up” or “prepare”) or as an intransitive verb (not taking an object, which would then be translated “act”). In Malachi 4:3, the same verb is used together with the same phrase (“the day that I”). In Malachi 4:3, this verb is clearly intransitive and must be translated as “on the day that I act.” Because of the proximity of these two similar phrases, it seems most reasonable to assume that the author uses the same verb in the same way. This also affects the way that we translate the entire verse. We favour the translation of the NIV2011, which renders Malachi 3:17 in this way: 'On the day when I act,' says the Lord Almighty, 'they will be my treasured possession.'
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.