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.

Was Satan created sinful?
What precedes the manifestation of sin ?
What is the evidence of genuine, living faith?
To another whose sight He had restored, what did Christ say?
What closed this terrible scene?
What trying experience did Christ then foretell?
Does death bring to Christians unassuaged sorrow?

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