What is Isra' and Mi'ray? Are these two names related with the Prophet (saw) ascension? Is it allowed to celebrate the "night of the ascension?". In some pages this celebration it is said that is a forbidden innovation to Islam. (http://islamqa.info).
Isra' is the spiritual night journey that Prophet took from Masjid al-Haram to Masjid al-Aqsa via supernatural means (that is without ordinary physical relocation), and Miraj is the spiritual journey he took therefrom to heavens until he reached the closet level to Divine Essence (dhat) which is the level of highermost affinity to Allah one can realize, and nobody (including jinns, angels or humans) has ever except the Prophet of Islam.
As for celebration, Islamic schools of jurisprudence differ. Salafists usually don't have a positive view of any form of celebration not explicitly endorsed in Islam, while Shias pretty much celebrate every positive event in history of Islam. Have a look at this similar Q&A for the reasons and roots of this difference.
Al Isra Wal Miraj was a physical journey to Al-Quds (Jerusalem) and to the heavens and finally to the arsh of Allah. It was unanimously agreed upon by reputable scholars that it was a physical journey involving the body.
that was a physical journey from the house of "umme_hanni" towards Jerusalem and to the heavens and finally to the "arsh of Allah". the TIME DIMENSION itself proven that it is physical quantity.So it must be stopped when we move with the speed of light or more faster. It proves the physical journey of Hazrat Muhammad (SAW). Long narration, but when Prophet (SAW) returned back home, the CHAIN on the DOOR, that prophet removed, was still MOVING, TO and FRO motion, the Water used for ablution still running on the ground.All the scholar agreed on this phenomena that it was a physical journey.The heart of Prophet (SAW) also washed . if took example of ADAM (AS) that one point we learn , they came from Heaven to Earth . so scientifically TIME travelling is possible .To understand this phenomena you must read article of """Time Travelling""". this made my belief more and more stronger.