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 was foretold of His treatment while on the cross?
Whose destruction was made certain by the death of Christ ?
Whose sovereignty is thus challenged?
What charge was made against Stephen concerning his attitude toward the ceremonial law?
What encouragement is given to heed this message?
In ministering to the needy, whom are we really serving?
Before Gentiles become Israelites, in what condition are they?

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