Greetings to you in the name of our Lord Jesus Christ.
Could you please clarify whether the current days are real from the beginning? Being Christians, we worship on the first day of the week. How do we know whether this is the same day of Jesus' resurrection? As several times calendars have changed in history. Have the days not changed?
At the time Jesus lived on this earth, the calendar in use was the Julian. It's "Saturday" aligned with the Sabbath (seventh day of the week) that the Jews kept. This is shown in the Gospels. Thus, we don't need to concern ourselves with the calendars prior to Jesus' time.
The Julian calendar was used for the next 1500 years, but it had a flaw. It was short roughly fifteen minutes every year. By 1582, the calendar was off by 10 days from the rotation of the earth. Pope Gregory issued a proclamation changing the calendar. Thursday, October 4, 1582, was followed by Friday, October 15, 1582. In other words, the number assigned to the day was changed, but the weekly cycle of the days of the week was not interrupted. England and the United States changed to the Gregorian calendar in 1752. It is currently the calendar used worldwide.
Therefore, we have the same cycle of seven days as existed in Jesus' day.
Thank you very much for the clarification brother.