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.

Who, therefore, did the speaking through these men?
For what purpose was Christ manifested?
What is the value of merely outward forms?
While drinking the wine, what gods did they honor?
Where did Moses command the Levites to put the book of the law which he had written?
By what means were the three Hebrews protected while in the fiery furnace?
What is one good evidence of genuine repentance?

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