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.

In the experience of sanctification, what attitude must one assume toward the truth?
Why was Christ's preaching so impressive?
5. In what instruction to His disciples did Christ recognize the existence of the Sabbath long after His ascension?
What were the names of these seven churches?
What was to take place under the third trumpet?
By what are we to try them?
What did the prophet Isaiah predict of Christ?

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