The 24 hour period from sundown tomorrow (September 25th) until sundown on Wednesday is the holiest day of the Jewish year; Yom Kippur, or the Day of Atonement. As God established it, the day was to be one of fasting, sacrifice and denial in atonement for ones’ sins throughout the past year. He admonished His people to “afflict your souls.”

Until Jesus Christ laid down His life as a sacrifice for mankind to make Atonement for our sins, we were cut off from God. Every year, on the Day of Atonement, the high priest would have to go through a very elaborate and bloody ritual which was a representation of our true High Priest, and His sacrifice to come.
 

[Source: YourSpiritMindandBody.com]

 
Because the ultimate sacrifice was provided for us by God Himself in the form of His son, Jesus Christ, the thinking among many Christians is that no further atonement for our sins is necessary. Yet while the New Testament clearly abrogates Old Testament ceremonial law, the Day of Atonement is still one that remains relevant as an important day of reflection and repentance.
 
At YourSpiritMindandBody.com Mike Shanta, a Christian, shares his reasons for observing Yom Kippur ~

The Day of Atonement is a fast day in which no food or water is consumed from sundown to sundown. It is a time of repentance, and to reflect on the great sacrifice of our High Priest, Jesus Christ. When you go 24 hours without food or drink you realize just how temporary and weak our bodies are. It is a time of solemn reflection on our short comings and areas we need to improve on. But most of all, it is a time to be humbly grateful for Christ’s sacrifice.
 
 

I have been keeping the Day of Atonement for around 30 years now, and will be doing it again beginning tomorrow evening at sunset. Every year I gain a deeper understanding of the significance of this day, and I become more grateful and dependent on our Savior because of it.
 
There are those who would argue the keeping of these days is not necessary anymore. As I said, it is not my place to judge or debate, only to share. I would not consider this a salvation issue, but a learning and relationship deepening opportunity. I believe if keep this day, you will be spiritually impacted by it.
 
If you should decide to try keeping the The Day of Atonement, let me say this. If you have any health issues, talk to your doctor before doing so. You should take any medications that you’re on with a little water. This day is not an exercise in the letter of the law, but is a day of rich symbolism and meaning that can draw you closer to God.

Although it was a bull, a ram and a goat that were ceremonially sacrificed on the Old Testament Day of Atonement, this day represents and commemorates Christ’s sacrifice which reconciles us to God through His atoning or covering of our sins.