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 is proved by any attempt to be justified by the law?
By what other title is the Comforter designated?
How was the prophecy given?
14. How was the sanctuary itself to be cleansed, and how were the sins of the people to be finally disposed of?
What does He say He will bring with Him when He comes?
What shows that the angels sent to Abraham and Lot were real beings?
How does God regard the afflicted?

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