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.

13. What three general divisions did Jesus recognize as including all the writings of the Old Testament?
To whom did this promised seed refer?
Whose workmanship is the Christian?
Upon what basis has Christ taught us to ask forgiveness?
What admission did they secure from Him as the basis of condemning Him?
What, therefore, did God say He would make?
What is the one great feature by which the true God is distinguished from all false gods?

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