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 one direct purpose of the incarnation of Christ?
How has Christ made it possible for righteousness to be imputed to the believer?
What will be the burden of the closing gospel message?
In what way was Jonah a type of Christ?
What relation does the Church of Rome sustain to other apostate churches?
How does Paul again express this same truth?
What imperial law was issued in AD. 386?

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