How did this change in observance of days come about, suddenly or gradually?

Answer

Gradually.
NOTES - "The Christian church made no formal, but a gradual and almost unconscious
transference of the one day to the other." 'The Voice From Sinai," by Archdeacon W. F. Farrar page 167.
This of itself is evidence that there was no divine command for the change of the Sabbath.

14. How was the sanctuary itself to be cleansed, and how were the sins of the people to be finally disposed of?
What else did Jesus do on the Sabbath?
Will this he a time of judgment?
Under what figures is the first trumpet described?
What will be the final doom of Satan and his angels?
Does God willingly afflict the children of men?
What is God called in the Scriptures?

Questions & Answers are from the book Bible Readings for the Home Circle