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 answer did he return when asked concerning his mission?
What kind of faith avails with God?
Was it possible for Christ to be holden of death?
What was done with the blood of the offering?
How was the overthrow of ancient Babylon foretold?
In what condition did David expect to rise?
To what is the life of the redeemed compared?

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