Fasting the Holy Month of Ramadan is the Forth Pillar of Islam Mandatory to every Muslim men and women who is Adult, sane, and healthy. Excused from fasting are those who can not fast due to old age or incurable sickness, they must pay ransom called fidiyat.
While those may not able to fast due to sickness or traveling, after the holy month of Ramadan, they must fast days they missed without fail before another Ramadan Comes.