The story behind celebrating bada mangal in Lucknow goes like this. Begum Janab-e Alia (wife of the third Nawab of Awadh, Shuja-ud Daulah 1753-1775), dreamt of a divine presence commanding her to build a temple honoring Hanumanji. The dream pointed her to a specific site where an idol of Hanumanji was buried. When they dug up the place they actually found a statue buried there.
The queen ordered that the statue be bought to the city with complete fanfare on an elephants back. The procession started but when it reached a particular spot the elephant stopped and would just not budge.
A temple was erected there of Lord Hanuman and month long celebrations happened across the city on all Tuesdays. The tradition now continues for more than 250 years and in its history Lucknow has never had Hindu Muslim riots ever.
One unique thing that you will find is on the top of the temple there is an insignia of a half moon to denote that the temple was built by the Muslim nawabs as a gratitude towards Hanumanji.