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 was predicted of Christ's preaching?
How do the children of God walk?
13. To the woman who had been healed by touching His garment, what did Christ say made her whole?
14. What instruction did Abraham give his servant Eliezer when sending him to select a wife for his son Isaac?
In what were the laws or commandments respecting sacrifices and burnt offerings written?
What class only have part in the first resurrection?
What promise has God made to bereaved mothers?

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