Fasting the day of doubt is haram because the Prophet Muhammad (peace and blessings of Allah be upon him) forbade the Companions to fast it

`Eid Al-Fitr is just one day on which it is prohibited to fast. As for the second and third days of Shawwal, it is not prohibited to fast on those days. It is permissible to fast it to make up a missed Ramadan fast or as a voluntary fast.

Fasting on the two days of Eid is haram. It is also haram to fast on the days of al-Tashreeq which are the three days after Eid al-Adha.

It is recommended to fast nine days of Dhul-Hijjah. The tenth day of Dhul-Hijjah is the day of ‘Eid which is prohibited to fast. What is meant by the virtue of fasting the ten days of Dhul-Hijjah is fasting nine days only, even though they are called ten days.

It is not allowed to anticipate Ramadan by fasting one or two days before it begins for those who do not have a habitual pattern of fasting or who are not continuing to fast after starting to fast before (in the first half of Sha`ban).