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.

To whom did Christ become the author of salvation?
Did Christ know beforehand the treatment He was to receive?
In what plain words did Christ foretell His resurrection?
What threefold union is thus established?
Did Christ come to abolish or to destroy the law?
What will Christ do with the saints when He comes?
After accepting Christ, what are we to do?

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