There is a slight difference between the Sabbath as observed by Jews and the Sabbath as observed by the Church of God. Jews observe the Sabbath from Friday evening to Saturday evening, while the Church of God observes the Sabbath from Saturday morning to Sunday morning. 

The reason there is a time difference between the Church of God and the Jewish Sabbath is that the founder of the Church of God, Mr. Ahnsahnghong Ahn, mistakenly believed that a day in the Bible begins in the morning when the sun rises, and this became the official doctrine of the Church of God. According to the Bible, a day begins in the evening when the sun sets, and the following evening is counted as a day.  This is because God established days and months based on the moon.

[Exodus 12:2] "This month is to be for you the first month, the first month of your year. Tell the whole community of Israel that on the tenth day of this month each man is to take a lamb for his family, one for each household. 

According to the Bible, Israel created a Jewish calendar based on the moon, and they celebrated the first day of the month by instituting a new moon ritual on the first day of the month.

[Numbers 28:11 ] 'On the first of every month, present to the LORD a burnt offering of two young bulls, one ram and seven male lambs a year old, all without defect. 

In general, the shape of the moon changes based on a month (29.5 days), and the process of change is as follows.​

New moon (1st of every month) -> Half moon -> Full moon -> Half moon -> New moon (1st of every month) 

Therefore, the standard of a month in the Bible starts with the new moon and ends the day before the new moon rises. Because of this, the reference point for the date change was a moonlit evening. The Jews changed the date standard on the evening of the rising moon, and according to this standard, they kept the Sabbath from Friday evening. 

God instituted the Day of Atonement on July 10 of the Jewish calendar, but He told us to observe it beginning on the evening of July 9.

[Leviticus 23:27~32] "The tenth day of this seventh month is the Day of Atonement. Hold a sacred assembly and deny yourselves, and present an offering made to the LORD by fire. .....(32) It is a sabbath of rest for you, and you must deny yourselves. From the evening of the ninth day of the month until the following evening you are to observe your sabbath." 

The reason why God commanded that the Day of Atonement be observed from the evening of the 9th until the following evening is because the date changes with the sunset, so the evening of July 9th would change the date to July 10th, so the Day of Atonement would be observed from the evening of the 9th. If we look at the six days of creation, we can see that the days begin with the sunset. If you look at the creation of the first day, God created light out of darkness, and the darkness was called night and the light was called day; therefore, the beginning of the day was a separation from the darkness.

[Genesis 1:4~5] God saw that the light was good, and He separated the light from the darkness. God called the light "day," and the darkness he called "night." And there was evening, and there was morning--the first day. 

God created light out of darkness on the first day, and called the light day and the darkness night, so the first day was 12 hours of evening (night) and 12 hours of morning (day), and the creation of the second day was also 12 hours of evening (night) and 12 hours of morning (day).

Therefore, the start of the day changes based on sunset, not sunrise. However, God's church does not know this secret, so they keep the Sabbath and all the feasts based on sunrise.