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.

Why was it necessary for Christ to go away?
In what spirit should children obey their parents?
How did Christ show His tender regard for children?
To whom does God reveal the secrets of the future?
Who did He say would enter the kingdom of heaven?
What did the Lamb do with this book?
7. What spirits stand ever ready to deceive and ruin those who are off guard or bent on going astray?

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