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.

With what measure of love should we serve others?
What advice was given to Timothy while preparing for the gospel ministry?
2. After being threatened with death if they did not make known the dream and the interpretation, what did the wise men say to the king?
What did the prophet see in this vision?
Who minister to God, and assist in the judgment?
Through what is the knowledge of sin obtained?
5. What did He say of those who should break one of the least of God's commandments, and teach men so to do?

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