' Shab-e-Besat ' is the eve of the deputation of the Holy Prophet (SAW) as a Messenger. It is the 27th Rajab which is also known as [coincides with] the 'Yawm ul Isra wal Mi'raaj' - the day of ascension.
BESAT
Meaning of BESAT in English
Advanced Islamic English dictionary. Расширенный исламский словарь английского языка. 2012