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 is one purpose for which the Holy Spirit was sent?
What contrast is drawn in the Scriptures between the Creator and false gods?
In what way was Jonah a type of Christ?
What is the beginning of wisdom?
Where do all go at death?
What is God called in the Scriptures?
Why did not these ancient worthies receive the promise?

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