… The Sabbath was not changed. It’s still on Saturday. The Lord’s Day is on Sunday. Christians moved the day of rest and worship from the Sabbath (which is the last day of the week) to Sunday (which is the first day of the week) in remembrance of Easter Sunday.
As far as the Calendar is concerned, God did not provide us a calendar, nor did he command when celebrations be held. Man created the calendar and thus created the liturgical calendar. Therefore, we can change the liturgical calendar as needed to properly express the Faith.
Additionally, God is Truth. Therefore, why should we have to revert back to using a calendar which cannot accurately keep track of time? Where is Truth in an incorrect date?