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.

Why were many attracted to Christ?
18. What is said of those who live to see the deliverance from modern Babylon, and the restoration of the true gospel?
What is necessary where there is a covenant?
What song will they sing?
What was one principal object in calling this council?
What is to be their fate?
What will be the difference between the portion of the righteous and the wicked?

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