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 thoroughly will the effects of sin he removed?
What is the result of godly sorrow?
What was to he accomplished at the close of the seventy weeks?
Of what sanctuary, or tabernacle is Christ the minister?
9. In what statement to the woman at Jacob's well did Jesus intimate that the ceremonial system of worship would be abolished?
Did Christ's followers keep the Sabbath after His death?
What encouragement is given to heed this message?

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