Praise be to Allah.

Fasting on the day which one believes to be the day of the Miraaj is not permissible and comes under the heading of Bidah. Even if a person is not sure, but he fasts this day for the sake of being on the safe side, it is as if he is saying, ‘If it is really the day of the Miraaj, then I will have fasted it, and if it is not, it will still be a good action that I have done, and if I will not be rewarded for it then I will not be punished.’ This attitude means that a person is committing Bidah, and he is a sinner who deserves to be punished. But if his fast is not because it is the day of the Miraaj, but is rather because it is his habit to fast alternate days, or to fast Mondays and Thursdays, and that happens to coincide with the day known as the day of the Miraaj, there is nothing wrong with him fasting it with that intention, i.e., the intention of fasting on Monday or Thursday, or a day on which he usually fasts.

If the Prophet (peace and blessings of Allah be upon him) taught us this acts of worship, then where is the evidence (daleel) for singling out the day of the Miraaj day for fasting? If there were any evidence that it is prescribed to fast on this day, no one would be able to say that fasting on this day is Bidah. But what is apparent is that those who say this mean that fasting is an act of worship in general terms, so that if he fasts he has done an act of worship for which he will be rewarded, so long as it is not on one of the days when fasting is not allowed, such as on Eid.

This would be correct if the person who is fasting did not single out a day which he believes is a day of virtue, such as the day of the Miraaj. What makes this the matter of Bidah is the fact that one is singling out this day. If there was any virtue in fasting thsi day, the Prophet (peace and blessings of Allah be upon him) would have fasted on this day, and he would have urged us to fast.

It is known that the Companions of the Messenger of Allah were more keen to do good than we are; if they had known that there was any virtue on this day, they would have fasted. Since we find no reports to that effect from them, we know that this is an innovated Bidah, and the Prophet  (peace and blessings of Allah be upon him) said:

“Whoever does an action that is not a part of this matter of ours (Islam) will have it rejected,” i.e., it will be thrown back on the one who does it"

Fasting on this day is an action which we find no report of the Prophet (peace and blessings of Allah be upon him) enjoining, so it is to be rejected.

And Allah knows best.

(Source - Islam Q&A)