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.

How were the Scriptures given?
Whom did Jesus pronounce blessed?
Will the Sabbath be observed as a day of worship in the new earth?
To whom does Paul say a crown of righteousness is promised?
Who are to share this fate?
To whose authority are the angels subject?
What does Daniel say of this kingdom?

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