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 faith declared to be?
What promise is made to those who repent and are baptized ?
What assurance has the believer of his union with God?
What is the only way sinners may he justified, or made righteous?
And what besides a mere belief in the truth is necessary?
How has God revealed to us the hidden things of the kingdom?
What special blessings were conferred on the Israelites?

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