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.

Which are some of the most touching and soul-winning of Christ's parables?
6. Under the influence of her tender care and faithful instruction, what is said of the child life of Jesus?
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?
When tempted to bow down and worship Satan, what reply did Christ make?
What will God's faithful servants be doing at this time?
What did the Sadducces in Christ's time deny?
What is to become of our earth in the day of the Lord?

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