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?
14. What beings of a higher order than man desire to study the truths revealed in the gospel of Christ?
What is Christ's relation to this faith?
What kind of worship results from false teaching?
Against what are husbands cautioned?
What is Satan himself called?
What warning has been given us through the Apostle Peter?

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