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 Christ's relation to this faith?
For what did David pray?
What is one of the offices of the moral law?
19. What testimony does Eusebius (AD. 270-338), a noted bishop of the church, a flatterer of Constantine, and the reputed father of ecclesiastical history, bear upon this subject?
When was there a wonderful darkening of the sun?
How does James say the rich have treated the just?
With what are we to comfort one another?

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