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 other book of the Bible is especially commended for our study?
Of what were the different parts of the image composed?
And how may we know that we love the brethren?
How early was this tendency manifested?
What does He indicate as one of its purposes?
7. What spirits stand ever ready to deceive and ruin those who are off guard or bent on going astray?
When will immortality be conferred upon the saints?

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