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 His teaching, to what did Christ direct attention?
What was indicated by the mixture of clay and iron in the feet and toes of the image?
What was the effect of this apostasy?
How did the Israelites in the wilderness on the sixth day prepare for the Sabbath?
Who is this dragon said to be?
How does the Sunday law of Charles 11, of 1676, read?
What does the Psalmist say about the dead praising God?

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