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 does God's character preclude Him from doing?
How is the fulfilment of this prophecy recorded?
What replies does Inspiration return to these inquiries?
What saved the people from the predicted overthrow?
What was to he the character of the little horn?
Of what was this memorial to he a sign?
What will be the third plague?

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