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.

9. In what words to Peter did Jesus indicate the kind of service a converted person should render to his brethren?
How should all Christian parents bring up their children?
22. How many of the commandments is it necessary to break in order to become a transgressor of the law?
What will He say to those on His right hand?
How long do the other class wait after the first resurrection before they are raised?
What shows that the actions of men are recorded?
What universal chorus of praise will then be heard?

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