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.

How did He encourage His disciples to look for the fulfilment of prophecy?
24. In what prayer for those who crucified Him did Christ manifest the true spirit of the gospel-love for sinner?
What only does Christ recognize as proper ground for dissolving the marriage relationship?
Who is responsible for the present state Sunday laws of the United States?
7. What, according to Matthew's account, did Christ say were to he the signs in the sun, moon, and stars?
What great change will then take place in their bodies?
Amidst poverty, suffering, and distress, what kind of workers does God wish to see?

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