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.

Who awakens the soul to a sense of its sinful condition?
In what song of praise did a host of angels join?
What peaceful, happy conditions will prevail in the next world as compared with those of this life?
How are the judgments upon Jerusalem again foretold?
10. What does the Psalmist say was the reason why God brought His people out of Egypt, and placed them in Canaan?
By whom were all things created?
Whatever may come, what blessed assurance has everyone who loves God?

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