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 marked the climax of Babylon's apostasy?
Through what is the knowledge of sin obtained?
What does Malachi say of those that fear the Lord?
What does He indicate as one of its purposes?
19. Just before the pouring out of the plagues, what call does God send to His people still in Babylon?
For what did Job say he would wait after death?
When Christ sent out the seventy, what did He tell them to do in the cities whither they went?

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