Iran is preparing for one of the largest state funerals in its history as ceremonies for Supreme Leader Ali Khamenei begin on July 4. Officials expect millions of mourners across Tehran, Qom, Mashhad, Najaf and Karbala, with Tehran alone forecasting up to 20 million participants. The massive security operation comes as Mojtaba Khamenei faces his first major test as Iran's new Supreme Leader. Will he make a public appearance and lead the funeral prayers? Here's what to expect from Iran's six-day farewell and why the event carries huge political significance.