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 necessary is faith?
How necessary is the experience of sanctification?
In what prophecy did Christ recognize Daniel as a prophet?
6. Until what time were the saints, times, and law of the Most High to be given into the hands of the little horn?
What power was represented by the ram having two horns?
What did John say was to be the end of this beast?
What did John the Baptist say to the Pharisees and Sadducees who came to his baptism?

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