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 led to Satan's sin, rebellion, and downfall?
In what does God delight?
22. In what announcement in the New Testament is the establishment of the kingdom of God made known?
What is the love of God?
Under the old covenant, what did the people promise?
When and by whom was the Council of Nice convened?
How do evil spirits sometimes treat those thus possessed ?

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