In Leviticus 23 God told the ancient Israelites to afflict their souls on the Day of Atonement, which occurred on the tenth day of the seventh month. God says:
Also the tenth day of this seventh month shall be the Day of Atonement. It shall be a holy convocation for you; you shall afflict your souls, and offer an offering made by fire to the LORD. Leviticus 23:26
The word in Leviticus 23:26 translated as "afflict" is translated as "humbled" in Psalm 35:13 when David wrote:
I humbled myself with fasting.
Thus the ancient Israelites understood the term "afflict your souls" to mean humble your soul and one of the ways that they did this was to fast as evidenced by David's fast. When we fast we deny ourselves something. Generally a fast refers to denying ourselves food for a certain period of time, but fasting can be applied to refraining from certain activities as well.
As the ancient Israelites observed the Day of Atonement they would fast from food to afflict their souls. But by Isaiah's day God did not seemed to be to pleased with their fasting. He said:
5 Is it a fast that I have chosen,
A day for a man to afflict his soul?
Is it to bow down his head like a bulrush,
And to spread out sackcloth and ashes?
Would you call this a fast,
And an acceptable day to the LORD? Isaiah 58:5
The ancient Israelites must have noticed God's displeasure in their fasting because they said to God:
"Why have we fasted,’ they say, “and You have not seen?
Why have we afflicted our souls, and You take no notice?’
‘In fact, in the day of your fast you find pleasure,
And exploit all your laborers. Isaiah 58:3
Apparently the ancient Israelites were not truly humbling themselves while they fasted. They were still seeking pleasure for themselves and taking advantage of people that worked for them. When God asked them to afflict their souls He wanted them to learn as they humbled themselves to think less about their own desires and more about the needs of others. God says:
6 “Is this not the fast that I have chosen:
To loose the bonds of wickedness,
To undo the heavy burdens,
To let the oppressed go free,
And that you break every yoke?
7 Is it not to share your bread with the hungry,
And that you bring to your house the poor who are cast out;
When you see the naked, that you cover him,
And not hide yourself from your own flesh? Isaiah 58:6,7
By asking the ancient Israelites to deny themselves on the Day of Atonement He had intended for them to be more considerate of others. But even though the ancient Israelites denied themselves by fasting they did not become more concerned about others. This angered God. Their self-interest during the Day of Atonement was especially wrong because it was during this time that they were supposed to set aside their own desires and think about others just as Jesus did during His life and ministry.
As a a result of the misuse of the Day of Atonement God said through the prophet Isaiah:
1 “Cry aloud, spare not;
Lift up your voice like a trumpet;
Tell My people their transgression,
And the house of Jacob their sins." Isaiah 58:1
According to Daniel 8:14, 9:25 and the application of prophetic year-day principle found in Ezekiel 4:6 we have been living in the anti-typical Day of Atonement since October 22, 1844. If the ancient Israelites were guilty of misusing the Day of Atonement and not honoring God are we as modern Israelites guilty? If Isaiah was told to cry aloud to the ancient Israelites and tell them their sins should we as God's spokesmen and women do the same?