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.

Who alone went into the second apartment, how often, and for what purpose?
How many of the dead will be raised?
When was there a wonderful darkening of the sun?
How did David express himself on this point?
With what do these trumpets deal?
What reason did Paul give for glorying in tribulations?
To whom must one belong in order to be Abraham's seed?

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