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.

In what way is this comparison further explained?
What other prediction asserts the same great truth?
What fate awaits blind teachers and their followers?
25. When Christ has finished His priestly mediatorial work in the heavenly sanctuary, what decree will go forth?
How is the church at the first advent described?
How does Paul give expression to this hope?
What is one evidence that one has a knowledge of God?

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