To turn over a new leaf is to make a fresh start and behave better. Time to turn over a new leaf, turned over a new leaf, promise to turn over a new leaf.
The leaf is a page in a book rather than anything on a tree, which almost nobody realises. It belongs to New Year and to the end of trouble, and it is nearly always said hopefully.
1phraseturn over a new leafTo make a fresh start and behave better.
“After failing his exams, he decided to turn over a new leaf and study every evening.”
turn over a new leaf is a reforming.
From leaf in the sense of a page in a book, so the phrase means starting a clean page.
Figurative by the same metaphor as with turn the page.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms