Assalamo alaikom, Fatwa: Is it permissible to change your mind and eat in a fasting nathr or nuthur day and fast it another day instead? Jazakom Allah khairan

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

It is an obligation for whoever vows to fast to do so, and it is not permissible for him to break the day that he vowed to fast after having started it except with a sound reason, whether one vowed to fast a particular day or just vowed to fast any day, and if one breaks the fast of this day he is obliged to make it up.

The Fiqh Encyclopedia reads: “According to the agreement of the jurists it is not permissible to interrupt an obligatory act of worship without a sound reason after having started it, because breaking it up without a sound reason is misconduct that contradicts the sacredness of the act of worship and it is forbidden to break up an act of worship.’’ Allaah Says (what means):  {… and do not invalidate your deeds.}[Quran 47:33] However, it is permissible to interrupt an act of worship while having a sound reason for doing so. For instance, a person may break the fast in order to save a drowned person, or out of fear for his own self, and so forth.”

Moreover, if the day that one vowed to fast is a particular day and one broke fasting it, then he is obliged to make it up and he is also obliged to expiate (for not fasting that particular day) according to the view of some jurists .

Allaah Knows best.