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.

Where was the Messiah to he born?
What requirement was made by these teachers from Judea concerning the ceremonial law?
Did Christ's followers keep the Sabbath after His death?
How ancient is this doctrine of Christ's coming?
How long was Jerusalem to be trodden down of the Gentiles?
Why did God give His only begotten Son to the world?
How many does Paul say will suffer persecution?

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