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.

What did He say a little later to the eleven?
What is declared to be one purpose of Christ's death?
How did the devil seek to induce Jesus to worship him?
What is one thing for which God has given the Sabbath to he a sign?
16. What striking testimony is borne by Neander, the noted church historian, regarding the origin of the Sunday Sabbath?
What power mentioned in the thirteenth chapter of Revelation is to enforce this mark?
What day does the commandment say is the Lord's?

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