The fourth day of the Hindu lunar calendar every month is observed as a fast to Lord Ganesh. Ganesh is 'Mangalkari' or auspicious. He is first amongst all Gods and prayed to first. He is also known as 'Vinayaka' or 'Vighnaraja' or one who removes all obstacles and impediments in the path of completion of any task or work.
During the annual festival (fourth day after new moon in the Hindu month of Bhadrapada) of Ganesh Chaturthy, people (especially in Maharashtra) celebrate this occasion with great fanfare, gaiety and piety. People establish small and big statues of Lord Ganesh in their homes, offices, and public puja places. 'Puja' and 'Archana' or prayers is offered to the lord and people fast, chant Mantras and offer 'modak' or ladoo sweets to Ganapati Maharaj.
The annual Ganesh Chaturthi is believed to be the birthday of Lord Ganesh. Ganesh is the son of God Shiva and Goddess Parvati. Parvati made Ganesh with the 'ubtan' or exfoliating paste from off her body and gave it life. Ganesh was assigned the task, by his mother, of gate keeper while his mother bathed. Shiva unaware of this wanted to enter his home but was not allowed entry by Ganesh. Shiva, in a rage, severed the head of the child. On hearing the noise, Parvati appeared and on seeing her beheaded child, demanded that the child be brought back to life. On hearing all the story, shiva replaced Ganesh's head with an elephant head and brought the child back to life.
Ganesh is the God of wisdom and prosperity and is usually depicted as sitting next to Goddess Saraswati and Goddess Lakshmi. Ganesh is also 'Shubh' or auspicious and all major work is started with an invocation to him.