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 whose image was man created?
How should the truth ever be cherished?
What did the people say when they saw these things?
12. After the subjects of the kingdom have been determined by the investigative judgment, what is given to Christ?
12. What universal boycott is to be employed, in an attempt to compel all to receive the mark of the beast?
Why did God destroy the antediluvian world?
What description is given of Gabriel in Daniel?

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