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.

To whom are those who suffer exhorted to commit their souls?
What reply did the Lord immediately make?
What restriction did God make respecting marriages in Israel?
What only does Christ recognize as proper ground for dissolving the marriage relationship?
Who was the last Babylonian king?
Did Christ come to abolish or to destroy the law?
Why, under these plagues, does the Lord give men blood to drink?

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