Why am I reciting Bhagavad Gita As It Is to you ? When it was told and to whom ? How is it related to Mahabharat ?

Why did Brahma Ji steal calves and cowherd boys in Vrindavan? - Shrimad Bhagavat Purana

It's from the book 'Shri Krishna', i.e. a summary of Tenth Canto of Shrimad Bhagavat Purana, an eternal or Sanatan scripture written by Sage Ved Vyas, literary incarnation of Lord Vishnu as per Vishnu Sahasranama. It also contains the most widely read worldwide 'Bhagavad Gita "As It Is"' spoken by God or Lord Krishna himself without any interpretation of commentator or author. 
The books are authored by His Divine Grace A.C. Bhaktivedanta Swami Srila Prabhupada, founder Acharya of ISKCON, International Society for Krishna Consciousness. You can purchase books online and at any ISKCON Centre.