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 in the early church showed living faith?
What did He thus abolish?
Then by whom was the Sabbath made?
What did He say should be done with every plant not planted by God?
How many of these beings did John see around the throne?
Upon what conditions are we promised needed blessings?
Without this faith, will God answer prayer?

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