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 prophecy foretold of the disposal of Christ's garments at the crucifixion?
Why was He to be named Jesus?
What was the result of this strife?
In what condition was the world before the Flood?
What was the result of this message?
Where did he say he would wait?
With what are the twelve foundations adorned?

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