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.

How fully did Israel once acknowledge their wrong doing ?
What does the fifth commandment require of children?
In what language is this clearly stated?
26. When Israel had apostatized, and were almost universally worshipping Baal, what appeal did Elijah make to them?
Under what figures is the first trumpet described?
What will be the third plague?
What is Satan himself called?

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