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.

Upon what conditions is an understanding of divine things promised ?
Where was the Messiah to he born?
What are children declared to be?
How did Christ Himself say He would come?
Upon what one fact does Paul base the Christian hope?
How are the people described who are developed by the threefold message here referred to?
What does Christ give His followers?

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