Ruling on Rajab 27 sawm

According to I'anathu thalibeen & hashiyathul jamal (Books of Islamic Jurisprudence) ; it is sunnah to fast on the night of mi'raj 

ويستحبّ صوم يوم المعراج ويوم لا يجد فيه الشّخص ما يأكله

(إعانة الطّالبين ٣٠/٢)

It is recommended to fast on the day of mi'raj and on a day when a person has nothing to eat .

( I'anathu thalibeen 2/30 )

ويسنّ أيضا صوم يوم المعراج ويوم لا يجد فيه ما يأكله 
حاشية الجمل ٢/٢٤٧

It is also sunnah to fast on the day of mi'raj and one the day when one finds nothing to eat.

 ( Hashiyathul jamal 2/246 )

But there is disagreement among imams regarding the exact day of mi'raj. All the different opinions regarding the date of mi'raj  doesn't have any priority or preference among the scholars.

Great scholar in shafi madhab ibnu hajar asqalani strictly opposes keeping fasting on the night of mi'raj and also said that hadees supporting this subject is not acceptable as evidance.

لم يرد في فضل رجب ولا في صيامه ولا في صيام شيئ منه معيّن ولا في قيام ليلة مخصوصة فيه حديث صحيح يصلح للحجّة

 (  تبيين العجب ٢٣ )

There is no authentic hadees has reported regarding the virtue of rajab or fasting on the month of rajab or keep fasting on a specific day or worshiping on specific night of that month.

 ( Thabyeenul ajab 23 )

Mulla aliyyul Qari ( r) said ; regarding this topic in sharahu shifa as:

قال السبكي الاجماع علي أنّه كان بمكّة واللّذي نختاره ما قاله شيخنا أبو محمّد الدّمياطي أنّه قبل الهججرة بسنة وهو في الرّبيع الأوّل قال ولا احتفال بما تضمّنه الحمدونيّة أنّه في رجب واحياء المصريّين ليلة السابع والعشرين منه بدعة 

 ( شرح الشّفا ١/٤٢١ )

Imam subuki said; All scholars have same opinion regarding mi'raj was happened in mecca. We have been following the view of Abu Abdulla al- dimyathi  his view was mi'raj was happened 1 year before hijra in the month of rabiyyul avval hamdoniyya said ; mir
'raj was happened in Rajab there is no evidence for that.

People of misr worshiped on the night of rajab 27 it is a bidath ( innovation)

  ( Sharahu shifa 421/1 )

In short, fasting on Rajab 27 in connection with night of mi'raj is not sunnah but it is innovation (bidath).

If a person fasts on that day without thinking that ,there is special virtual and merits on it; then there is no problem in fasting.