The Sabbath Command as a Holy resting day given by our Father God YHWH is found in Exodus 20:8-11 - https://biblehub.com/parallel/exodus/20.htm. It is the 4th Commandment written by the finger of Father YHWH and given to Moses on Mount Sinai (Horeb) in Saudi Arabia.
The 10 Commandments according to the Judean scriptures:
In Exodus 16:1-36 - https://biblehub.com/parallel/exodus/16.htm we learn that Sabbath is from sunrise until sunset (12 hrs) on the seventh day of the week.
The sun, moon and stars was created by Father YHWH on the 4th day according Genesis 1:14-19 - https://biblehub.com/parallel/genesis/1.htm to separate the days from the nights, and let them be for signs, seasons, hours, days, months and years.
Sabbath calendar counting guide:
The Hebrew calendar does not have name days, but numbers. The new moon day is the 1th day of the new month and so when you start counting you count 1 as 1. Then you count 7 days forward and you have then found your first regular Sabbath day. For example, Tuesday March 21 2023 is day 1, that makes Monday March 27 2023 the first regular Sabbath day of the new month. Every Monday forward until the next new moon is then a regular Sabbath day.
The Eastern Orthodox Church aka the Roman Catholic Church and allies changed all of that. They made days into name days and moved the Sabbath to the Lord's day aka Sunday. In the pagan religious belief systems Satan is represented as the Sun god. So by moving the Sabbath to Sunday people was then officially worshipping Satan as their god by human created law. Our Father in Heaven YHWH has never changed the Sabbath days. Sabbath has from the beginning always been found by counting from the new moon days.
To find the correct new moon day we use the new moon day of Jerusalem. The reason for this is because the 3 Temples of God YHWH was situated there before their destructions.
To find the correct new moon day you need to use a lunar/moon calendar - https://www.timeanddate.com/moon/phases/israel/jerusalem
Use a sunrise and sunset locator to find the right sunrise and sunset times for your area - https://www.timeanddate.com/sun/
We do not take into account the GMT (Greenwich Mean Time) time zones because it was invented by the United Kingdom in the eighteen hundreds (but if you for example live above the polar circle, then use Jerusalem time zone, minus or pluss your GMT position).
Today the Temple of God is inside the people through the Holy Spirit who believes that Yahushua of Nazareth is the Messiah. Those people in the Bible are called Israel after Jacob and the word Israel means in Hebrew, Prince or Princess of God YHWH.
Thank you for your time, Father God YHWH bless you and yours more, and have a wonderful Sabbath Shalom!