The Day of the Lord
Throughout the Bible there are references to the coming Day of the Lord. Day can be used symbolically to reference a period in time, like the phrase "back in the day", but understanding that Scripture was written by the Holy Spirit with precision, symbolic terms like day also have precise meaning. In Scripture, day can reference time periods with differing orders of magnitude, but the magnitudes are precise and not approximate.
Below, verses containing day references are loosely grouped by similar phrases. Prophetic references in Scripture, like day, are often fractal and point to more than one scale of fulfillment. In general, Day of the Lord refers to the future thousand year reign of Jesus, visible bodily on earth, following the Great Tribulation, before this universe is destroyed exothermically and He creates a New Heaven and New Earth.
Peter specifically explains that a day can be a reference to 103 years:
2Pe 3:8
But, beloved, do not forget this one thing, that with the Lord one day is as a thousand years, and a thousand years as one day.
The Day
Zec 14:1
Behold, the day of the LORD is coming,
And your spoil will be divided in your midst.Zec 14:3
Then the LORD will go forth
And fight against those nations,
As He fights in the day of battle.
2Pe 3:10
But the day of the Lord will come as a thief in the night, in which the heavens will pass away with a great noise, and the elements will melt with fervent heat; both the earth and the works that are in it will be burned up.
2Pe 3:11
Therefore, since all these things will be dissolved, what manner of persons ought you to be in holy conduct and godliness,
2Pe 3:12
looking for and hastening the coming of the day of God, because of which the heavens will be dissolved, being on fire, and the elements will melt with fervent heat?
2Pe 3:13
Nevertheless we, according to His promise, look for new heavens and a new earth in which righteousness dwells.
In That Day
Zec 14:4
And in that day His feet will stand on the Mount of Olives,
Which faces Jerusalem on the east.
And the Mount of Olives shall be split in two,
From east to west,
Making a very large valley;
Half of the mountain shall move toward the north
And half of it toward the south.
The Third Day
Hos 6:2
After two days He will revive us;
On the third day He will raise us up,
That we may live in His sight.
Luk 13:32
And He said to them, “Go, tell that fox, ‘Behold, I cast out demons and perform cures today and tomorrow, and the third day I shall be perfected.’
In 1&2 Tim seems to a difference between latter times (1 Tim 4:1) and last days (2 Tim 3:1).