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.

How great. is Gods love for the world?
What indwelling Power keeps such from sinning?
What is the very life of the Spirit?
What challenge is made by those who worship the beast?
From what statement do we learn that these ordinances related to the sacrificial system?
How did Jesus answer them?
What shout of triumph was heard in heaven following the victory gained by Christ?

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