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 the ultimate purpose of faith?
What did the prophet say His name should be called?
By what is Grecia, the kingdom succeeding Medo-Persia, represented in the image?
What did Christ say of the certainty of this prophecy?
What is set forth as the basis for hope and comfort?
Who did he say he was?
23. Just preceding Christs second coming, what solemn decree will go forth showing that the cases of all have been decided ?

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