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 highly does God regard obedience?
What is said of his mother and his grandmother?
How was Moses taught during the days of his early childhood?
5. What, in the parable, did God say to the rich man who thought to build larger barns in which to store his goods?
Does Jesus sympathize with us in our afflictions?
For what purpose did God give His Son to the world?
Of what are the twelve gates composed?

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