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.

Of whom did Moses and the prophets write?
What home did God make for man in the beginning?
13. To the woman who had been healed by touching His garment, what did Christ say made her whole?
19. What message did Christ send to John the Baptist while John was in prison, to strengthen his wavering faith?
In the judgment-hour message, whom are all called upon to fear, glorify, and worship?
1. According to the words of the angel to Daniel, when might the world look for an increase of knowledge?
How numerous are these wicked spirits, or fallen angels?

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