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 the result of the Jews' not accepting Christ?
Whom does the Lord chasten?
What did the goat with the notable horn represent?
What system was established on account of man's transgression of the law of God?
Under the old covenant, what did the people promise?
What did Christ say was to be the next great event following these signs?
By what name are the members of this family called?

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