The Calendar
Has the calendar been changed? Yes, but the weekly cycle has never been changed. The Julian calendar was in use when Jesus was on earth. The calendar, which continued in use for 15 centuries was not accurate in length of its year, for it was a quarter hour too long. By 1582, it was 10 days off. Pope Gregory initiated a change in the calendar by going to the Gregorian Calendar, and to make up for the error in the Julian calendar, 10 days were added to the calendar. In October, 1582, Thursday the 4th was followed by Friday the 15th in Italy and a few other countries. England and America changed its calendar in 1752 and Russia finally in 1914. Yet the weekly cycle was never affected. During the time that England, Russia, and Italy had different calendars, Sunday, Monday, and Tuesday were always the same in each country. [As a side note, the beginning of the year began March 1st under the Julian calendar, whereas he beginning of the year begins January 1st under the Gregorian calendar].
We do the same thing today, because every four years we change our calendar; it's called "leap year." At the end of February, we add one day every four years to our calendar. But notice the days of the week never change, it is only the number of the day that changes. For example, when that extra day is added, it might go from Thursday the 28th to Friday the 29th; the days of the week remain the same, only the numbers of the day change. There is no difference when 10 days are added to the calendar as well.
History has exact calendar records going back to Julius Caesar, several decades before Christ, and the weekly cycle has always remained intact. We have exactly the same weekly cycle today as was used in Jesus' day, and Jesus said that the day then called the Sabbath by the Jews, the seventh day of the week, was His day, the true Lord's Day. Yes, the amount of days in a month has changed, the amount of days in a year has changed, the amount of weeks in a month has changed, the amount of weeks in a year has changed, the amount of months in a year has changed. But the amount of days in a week has never changed since man was first created on earth.