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.

In what language did Isaiah foretell Christ's birth?
What gives victory in our conflicts with the world?
What is one evidence of this change from death to life?
What did the prophet say His name should be called?
How may we know that we love the children of God?
7. What spirits stand ever ready to deceive and ruin those who are off guard or bent on going astray?
When Christ sent out the seventy, what did He tell them to do in the cities whither they went?

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