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 were the Scriptures given?
Besides the court, how many parts had this sanctuary?
How had they done this?
What does it mean when used with reference to law?
In His first recorded discourse, what did Christ say of the law?
1. Following the seven seals, under what symbols was the next series of thrilling events shown the Apostle John?
What is the last enemy to be destroyed?

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