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 specific instance is mentioned by Peter?
In what condition are all men?
12. Do Catholic authorities acknowledge that there is no command in the Bible, for the sanctification of Sunday?
When did the stars fall, as here predicted?
What will be done with the righteous living?
To what Old Testament promise did Peter evidently refer?
How readily will their wants be supplied?

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