Tome of Night and Day

If it's neither day nor night, it becomes day
as this artifact enters.

Whenever a player casts a spell, if it is day,
that player gains 1 life; if it is night, that
player loses 1 life.

{2}{w/b}, {t}: If it's night, it becomes day.
Otherwise, it becomes night. Activate only
as a sorcery.