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 explains God's apparent delay in dealing with Sin ?
What change is wrought in conversion, or the new birth?
18. What effect did Christ's miracles have upon the individuals restored, and the people who witnessed them?
After creating Adam and Eve, what did God say to them?
What is the essential teaching of the new covenant?
When the trees put forth their leaves, what do we know?
19. Just before the pouring out of the plagues, what call does God send to His people still in Babylon?

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