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.

What is the only unpardonable sin?
What will follow this union with Christ in His death and resurrection?
What condition is attained in obeying the truth?
What was to be the nature of the next kingdom after Babylon?
What scriptures show that Christ's coming will be a time of reward?
What answer did Elijah make?
What definite period is mentioned under this trumpet?

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