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.

Through whom are repentance and forgiveness granted?
21. Although Pilate had declared his belief in Christ's innocence, yet what cruel punishment did he inflict upon Him?
12. What prohibition did God give His chosen people against intermarrying with the heathen nations about them, and why?
In what other statement is this same truth expressed?
By what may we know that we have passed from death unto life?
Which commandment alone of the Decalogue reveals the true God and Author of the law?
What takes place at the sounding of the trumpet?

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