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.

In what way is this comparison further explained?
What is Christ's relation to this faith?
Have the worthies of old gone to their reward?
Under what striking symbol was all this foretold?
Why was the gospel to be preached to the Gentiles?
In what condition did David expect to rise?
Who will walk in the light of the city?

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