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 accompanies repentance?
What was He to take away in the midst of this week?
What change will then take place in both the living and the sleeping saints?
What were these martyrs represented as doing?
What striking figure is used to describe the destruction wrought under the second trumpet?
After whose body will these resurrected ones be fashioned?
What is Satan himself called?

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