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.

5. After the healing of the blind man, upon what charge did some of the Pharisees seek to prove that Christ was not of God?
What did the prophet say Christ would be called to endure?
Why should husbands be considerate of their wives?
How early did Hannah dedicate her son Samuel to God?
How was the fourth kingdom represented?
What warning has been given us through the Apostle Peter?
What peaceful condition will prevail in the earth made new?

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