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.

With whom was Christ crucified?
What was Christ made, to release men from sin?
What did He say to the two blind men as He healed them ?
What reason did Solomon give for urging all to fear God and keep His commandments?
Which day is the Sabbath?
What shout of triumph was heard in heaven following the victory gained by Christ?
Why did God give His only begotten Son to the world?

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