For Jews living in the Middle East, a day begins in the evening when the sun sets. Unlike the modern world, where the date changes at midnight, they define a day as the time between sunset and the next day's sunset. Here's why the day starts in the evening. 

1.Based on the six days of creation in Genesis, God established the beginning of the day as evening.

[Genesis 1:3-5] So God said, "Let there be light!" and there was light, and it was good, and God saw that it was good. God divided the light from the darkness, and called the light day and the darkness night. So the first day was night and the second day was day.

According to the Bible, when God created the heavens and the earth, there was only darkness (night). God created night and day by separating light (day) from darkness (night), so a day begins with evening (night) and ends with morning (day).

2. The Day of Atonement begins 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."

God set July 10 of the Jewish calendar as the Day of Atonement, but He said that the Day of Atonement began on the evening of July 9. Since the Jews change the date at sunset, the evening of July 9 becomes the 10th. so the start of the date change is at sunset. 

3. Start of the day through the book of Zechariah

[Zechariah 14:7] It will be a unique day, without daytime or nighttime--a day known to the LORD. When evening comes, there will be light.

Through Zechariah, God foretold a specific day, saying that it would be neither day nor night, but when it was dark, the eternal light would appear. According to Genesis, this day foretold God's day of eternal rest, and Zechariah testified that the day begins when it is dark. Therefore, the beginning of the day is evening.