The first part of Matthew 20 is often referred to as the 'Parable of the Vineyard Workers' which places the emphasis on the workers.

In Matthew 20:1 Jesus himself makes the point that the parable is to illustrate how the Kingdom of Heaven operates and the character of God (the landowner). Jesus places the emphasis on the landowner.

The Matthew 20 chapter break also appears unfortunate (made worse if reading this a chapter at a time electronically). One wonders about the history of these chapter / verse traditions. Useful for quickly referring to a particular section but maybe sometimes obscuring the intention of the author (and God).
For a brief history lesson see: Chapters and Verses of the Bible.
Anyway the connection between Matthew 19 and Matthew 20 is not the purpose of this post.

The Biblical day starts in the evening (repeated several times for emphasis in Genesis 1).
The Biblical working day being dawn to dusk (again in Genesis 1 we find God working during daylight - interesting comment by Jesus in John 9:3-5).
In round figures that averages out as a working day being from 6am to 6pm and we have Jesus referring to 3rd hour (9am), 6th hour (12pm), 9th hour (3pm) and 11th hour (5pm).

Jesus does not directly refer to 6am, the start of the working day, instead we find the landowner went out early in the morning, suggesting before 6am. He went out with the intention of finding those who were eager to work (2 Thessalonians 3:10)? Maybe some came from a place of dedication (Acts 16:14-15, compare with Revelation 14:6-7), others came from a place of desperation needing money and food (Acts 16:29-31).

Its interesting to notice that Jesus mentions that the landowner "agreed to pay them ..." which implies that there may have been some haggling. Maybe at this early time some hoped for more than a normal days wage for a normal days work.

The landowner went out again at 9am, 12am and 3pm, found some guys handling around doing nothing and sent them into his vineyard as well. Only a promise 'to pay what is right', the implication being that they might get a pro-rata rate, but even that is not guaranteed. There is no hint of condemnation for wasting time.

The implication from dividing the day into 3 hour slots is that the 3pm hiring was the last chance, but 2 hours later at 5pm (the 11th hour) another visit to the market-place and more are hired. Reminds me that God desires that no one loses out (1 Timothy 2:4) and His mercies never come to an end (Lamentations 3:22-23).

The purpose of the parable is to illustrate the generosity of God (Matthew 20:15).

It always contains a warning of the dangers of envy and not walking humbly with your God (Micah 6:8).

We also find Matthew 20:16 links back to Matthew 19:30 and the guy dedicated to working hard in the hope of gaining God's approval (Matthew 19:20). At least he was honest enough to ask the question 'what do I lack', although he was unwilling to accept the answer and went away empty (see Matthew 8:11-12). Doing the right stuff and asking the right questions will not get you into the Kingdom of Heaven but only doing what Jesus says "go and work in my vineyard (Matthew 20:3)', see John 6:29.