Some non-Muslims are not aware that there are two types of Muslims that are available right now, namely the Shia and Sunni Muslims. Sunni Muslims are known to be Muslims that will base the things that they do on Sunnah. These are teachings and other lessons that are written by their prophet Muhammad.
Shia Muslims are the ones who believe that the right successor has been chosen, especially after the Prophet Mohammad died. It is the Sunni Muslims, the smaller group of Muslims who believed that there was another person who should be the rightful successor of the prophet Mohammad. Shia and Sunni Muslims may celebrate certain holidays, even if they have different beliefs.