Adi Shankara was one of the most revered Hindu thinker/philosopher; he is also an incarnation of the Hindu god, Shiva. He is famous for allowing all sorts of criticism to his work and thoughts which he believed just made his philosophies better. He travelled the whole Indian subcontinent propagating his philosophy and debating with other philosophers on his thesis. All of his work is written in Sanskrit and has been influenced by the Vedas.
Sri Sankara Jayanthi is celebrated every year in dedication to Adi Shankara and is also the day of his birthday. It falls in the Vaishakha (April/May) month of the Hindu calendar.
Adi Shankara is one of the prime philosophers who led to the origin of Hindu laws and procedures. He and many others are the ones who set down the codes which have to be followed by every Hindu and are still in practice across India. Majorly, he taught Hindus how to worship the deities of heaven.
Since Adi Shankara is the person who formed the basis of the Hindu culture, this day is celebrated all over India with a lot of enthusiasm and many pray to him in order to express their gratitude.