Jamieson, Fausset and Brown on 2 Peter 3:12

12. hasting unto--with the utmost eagerness desiring [Wahl], praying for, and contemplating, the coming Saviour as at hand. The Greek may mean "hastening (that is, urging onward [Alford]) the day of God"; not that God's eternal appointment of the time is changeable, but God appoints us as instruments of accomplishing those events which must be first before the day of God can come. By praying for His coming, furthering the preaching of the Gospel for a witness to all nations, and bringing in those whom "the long-suffering of God" waits to save, we hasten the coming of the day of God. The Greek verb is always in New Testament used as neuter (as English Version here), not active; but the Septuagint uses it actively. Christ says, "Surely I come quickly. Amen." Our part is to speed forward this consummation by praying, "Even so, come, Lord Jesus" (Re 22:20).

the coming--Greek, "presence" of a person: usually, of the Saviour.

the day of God--God has given many myriads of days to men: one shall be the great "day of God" Himself.

wherein--rather as Greek, "on account of (or owing to) which" day.

heavens--the upper and lower regions of the sky.

melt--Our igneous rocks show that they were once in a liquid state.