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.

8. How did Christ show from the Scriptures that the promised Saviour of the world must be both human and divine?
What power was represented by the ram having two horns?
How was the ceremonial law made known to Israel?
How long did he continue this work there?
What did Christ say would be the moral condition of the world just preceding His advent?
What role do his agents assume?
What is an evidence of perfection?

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