What days of the week does Muslims have to fast?

It is the fourth Islam pillar (in sunni view of pillars of Islam) that a Muslim should fast the month of Ramadan (the 9th month on Islamic Calendar) unless there are licit reasons not to fast the month or part of the month.

However, voluntarily Muslims are encouraged to fast Mondays and Thursdays as well as the three days of the middle of the Lunar month (called alaiyam embid).