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 the experience of sanctification, what attitude must one assume toward the truth?
What did God say to Abraham concerning the Amorites?
On what day was John in the Spirit?
22. What did Augustine, the father of this theocratical or church-and-state theory, teach concerning it?
13. The righteous dead are raised at Christ's second coming. When will the rest of the dead, the wicked, be raised?
What did the Lamb do with this book?
Why was this to be?

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