Why are there 4 different days to celebrate the Son of God?

Why are there 4 different days to celebrate the Son of God? The Roman, Protestant, Greek, and Russian days. What about God's day and appointed hour?

https://www.tbgrace.org/school-of-faith/

https://www.schooloffaith.eu

https://www.tbgrace.org

https://www.biblicalexperience.com

https://www.dailytorahportion.com

https://www.tbgrace.org/restoring-haven/

https://www.tbgrace.org/women-empowerment/

https://www.jordanriverexperience.com