The phrase "Turn Over a New Leaf" is a figurative use of language which means to begin again with the intention to reform and start afresh. The reference may be to turn over a page which is clean and fresh. In interpersonal relationships, to "Turn Over a New Leaf" would mean to reform one’s behavior and behave in a more responsible and acceptable manner. "Turn Over a New Leaf” is an idiom which usually implies an improvement in one’s behavior.
The “leaf” in "Turn Over a New Leaf" may refer to the page of a book as you turn leaves while turning the pages of a book. It is derived from turning a page in order to start a new page of the book. So literally speaking "Turn Over a New Leaf" may mean to “turn to a new page.” In fact, in earlier times books were written traditionally on palm leaves, stitched together with the help of a cord. Since the 10th century it is common to refer to pages as leaves. However, figuratively the term "Turn Over a New Leaf" is basically an idiom which means to make a new beginning.
To "Turn Over a New Leaf" would imply that the concerned person has vowed to make changes in his life that would contribute to his overall improvement. The move may be towards making radical changes in one’s habits or behavior for the better. There is always scope for improving various facets of one’s life. What remains is to identify the weak areas and incorporate the necessary changes so as to turn over a new leaf by eliminating the weak traits. Though there is no specific time earmarked to “turn over a new leaf”, many people make New year resolutions to turn over a new leaf by promising to make beneficial changes in their lifestyle patterns.
The idiom "Turn Over a New Leaf" is not restricted in its usage to individuals. It can be used in general to refer to broader organizations or other such entities. A multi national corporation known for its rigid practices may incorporate friendly policies for its employees and may be described as turning over a new leaf. Similarly when radical changes are made to improve the governance of a state, it may be termed as turning over a new leaf for the benefit of its citizens. An educational organization can be known for turning over a new leaf when it introduces a child- friendly environment in its campus.