It is forbidden to do so because there are certain days of the year on which one is not allowed to fast. The Messenger of Allah said: "There is no [reward for] fasting for the one who perpetually fasts." This is related by Ahrnad, al-Bukhari, and Muslim.
If one breaks his fast during the days of 'id and the days of tashreeq, then his perpetual fasting would no longer be considered disliked. In his comments on this issue, at-Tirmidhi says: "A group of scholars dislike fasting every day if it includes the 'ids ['id al-Fitr, 'id al-Adha] and the days of tashreeq. If one breaks his fast on those days, his action is no longer disliked, as he is no longer fasting the whole year." The scholars are Malik, ash-Shaf'i, Ahrnad, and Ishaq.
The Prophet approved of Hamzah al-Aslami's nurnerous fasts when he told him: "Fast if you wish and break your fast if you wish." This hadith was mentioned earlier.
The Messenger of Allah forbade a woman to fast if her husband was present until he gave her his perrnission to do so. Abu Hurairah reported that the Prophet said: "A woman is not to fast [even] for one day while her husband is present except with his permission, unless it is during Ramadan." This is related by Ahmad, al-Bukhari, and Muslim. The scholars have interpreted this prohibition as one of forbiddance, and they allow the husband to end his wife's fasting if she fasted without his perrnission and he seeks his right [to sex] from her. This is also true, obviously, for days other than those of Ramadan in which case she does not need her husband's permission. Similarly, if she fasted without his permission because he was not present, he has the right to end her fast when he retums.
If the husband is sick or incapable of intercourse, it is permissible for the woman to fast without his perrnission--that is, it is similar to the case of where the husband is not present.