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.

In order to baptize the eunuch, where did Philip take him?
12. What universal boycott is to be employed, in an attempt to compel all to receive the mark of the beast?
How did the devil seek to induce Jesus to worship him?
How early was this sacrificial system known?
21. What miraculous event occurred at the death of Christ, signifying that the sacrificial system was for ever at an end ?
How is this beast further described?
What universal chorus of praise will then be heard?

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