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.

Why was He to be named Jesus?
What kind of church would He thus be able to present to Himself?
What is a \"wind of doctrine\"?
How did He encourage His disciples to look for the fulfilment of prophecy?
What are God's people told to do at this time?
In what is the wrath of God filled up?
What promises are made by Christ to the overcomer?

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