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 is the true basis of the brotherhood of man?
Unto whom will the saints forever ascribe praise?
What did Christ's words at Jacob's well lead the woman of Samaria to ask?
In the interpretation of the vision, how is this self exaltation set forth?
What did some of the people do on the seventh day?
What class of men did he say would arise in the church?
How long will they live?

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