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 were the Scriptures given?
What remarkable experience attended the baptism of Jesus?
What is said of Christ's childhood and early life?
Did Jesus ever make use of ordinary means in performing His miracles?
What provision has been made so that we may keep God's law?
What was given these martyrs?
Whom does God comfort?

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