1
general::
Prevent a misfortune, as in They had forgotten the knife to cut the wedding cake, but Elizabeth arrived with one and saved the day.
American Heritage Idioms
2
general::
Phrase(s): save the day
to produce a good result when a bad result was expected. • The team was expected to lose, but Sally made three points and saved the day. • Your excellent speech calmed the crowd and saved the day.
McGrawhill's American Idioms And Phrasal Verbs