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 contrast with the pride and self-exaltation exhibited by Satan, what spirit did Christ manifest?
6. Until what time were the saints, times, and law of the Most High to be given into the hands of the little horn?
For what purpose did the law enter?
What is it that makes a thing holy?
How, according to Luke, did Christ answer this question?
Where are the dead when they hear the voice of Christ calling them to life?
What does Daniel say of this kingdom?

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