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 another evidence of divine acceptance?
With what was He filled?
Where was Daniel at this time?
3. What urgent petition of the prophet connects this prayer with the vision of the taking away of the continual mediation and the desolation of the sanctuary recorded in Daniel 8?
What was the result of this message?
What is Spiritualism defined to be?
What is Jesus able to do?

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