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.

With what measure of love should we serve others?
What does Christ thus become to the believer?
What precious promise did Jesus make to His disciples shortly before His crucifixion?
What kind of teaching have men substituted for the words which are spirit and life?
What special reason did the Israelites have for keeping the Sabbath?
What did the first Sunday law enacted in America, that of Virginia, in 1610, require?
What will the righteous do upon rising from the grave?

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