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.

Does Christ possess these same attributes?
What is to be the fate of those who, instead of worshipping God, engage in this false worship?
What is said of the stability of God's character?
18. In what statement was Christ promised as a Savior and Deliverer of the race as soon as sin entered?
By whose Spirit did the prophets write?
7. What, according to Matthew's account, did Christ say were to he the signs in the sun, moon, and stars?
What did the prophet Isaiah predict of Christ?

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