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.

When was Jesus born?
By what means are sins washed away?
What is assured to the believer in Christ?
Does the Bible recognize what may be called \"present truth\"?
How are the present effects and future results of chastisement contrasted?
How was this covenant then confirmed and dedicated?
16. What action of the American Federation of Catholic Societies indicates that Catholics will gladly \"join hands\" with Protestants in enforcing Sunday observance by law?

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