In the whole year, the days on which fasting is not permissible are
the 2 Eids of Fitr and Adhha. It is Makrooh (disliked) to fast on the day of Ashura. It
is permissible to fast on all the other days of the year.
Besides the obligatory fast of the whole month of the Ramadhan
, in the other 11 months, it is recommended to fast on any 3
days although there is special mention in the books of narrations to
fast on the 1st and the last Thursday of the lunar month and on the
Wednesday of the middle 10 days of the month.
We also have Hadeeths that the Holy Prophet (SAWA) used to fast on
every Thursday and Monday.
Recommended fast will be not allowed if it contradicts with any obligatory like the rights of the husband on his wife. Wife is not allowed to fast a recommended fast if it harms her husband's rights.