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.

What did the Lord command Abraham to do?
Did Jesus ever make use of ordinary means in performing His miracles?
For what purpose is the manifestation of this one Spirit given?
What influence has this hope upon the life?
Why, under these plagues, does the Lord give men blood to drink?
Is this, for the time being, a source of pleasure?
Whom does God comfort?

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