It is the sunnah of the Prophet (PBUH) to fast on the 10th of Muharram (the Day of Ashurah), which Insha’Allah will be on Monday, September 9th this year. He (PBUH) also said that we should fast one additional day along with the 10th to set us apart from other religious communities. Therefore, we should fast on both Sunday, September 8th and Monday, September 9th AllahuAlim (and Allah knows best).