What is the ruling on observing fast on 26th and 27th of Rajab? What about making dhikr at night between 26th and 27th of Rajab?

Celebrating the night of mi’raaj is a bid'ah. There is no proof to specify the days of 26^th and 27^th Rajab with fasting or spending the night in dhikr. Any act of worship must fulfil two conditions:

1. It  must  be done sincerely for the sake of Allah
2. It must be according to the Sunnah.

If any act of worship does not fulfil these two conditions it is rejected.