Wiktionary
vb. (context idiomatic English) To rescue a person or situation from imminent danger or major failure.
Wikipedia
"Save the Day" was the first official single taken from Australian rock group, The Living End's debut self-titled album. It followed the release of the " Second Solution/Prisoner of Society" double A-side single the previous year. It was released in September 1998, spending 17 weeks in the Australian ARIA Singles Chart and peaking at No. 22. The song also reached No. 10 on Triple J's Hottest 100 for 1998. The single was later certified as Gold (selling in excess of 35,000 copies).
The single was also released on 7" vinyl however it was a limited release with only 500 copies issued.
Usage examples of "save the day".
By the time the cavalry comes to save the day, Ill be ready for grub.
Only a victory by Aiken over the ogrish Firvulag general, Pallol One-Eye, could save the day for the Tanu.
His idea was that you could send divisions forward or back to a battle you had lost, or were going to lose, and save the day.