Ganesha Chaturthi is one of the most important Hindu festival in India. It
is the day on which Lord Ganesha, the son of Lord Shiva and Parvati was
born. The day thus marks the auspicious birth of one of the most important
Hindu God. This day is also known as Vinayaka Chaturthi or Vinayaka Chavithi
in many parts of the country. It is celebrated on of fourth day of the
waxing moon period in the month of Bhaadrapada according to the Hindu
calendar. According to the Gregorian calendar, this day falls between 20
August and 15 September. The festival is celebrated with immense enthusiasm
in Maharashtra and Andhra Pradesh for ten days. If you want to explore the
origin and history of this festival, you can refer to the article given
here.

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Origin
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According to the legend, the festival marks the auspicious day of the birth
of Lord Ganesha. The story goes as follows- Lord Shiva, the Hindu God of
resolution, was away from Kailash due to some work. As Parvati was alone at
home, she felt the necessity of some one to guard the door to her house
while she took bath. When she did not get any one, she conceived of the idea
of creating a son who could guard her. She then created Ganesha out of her
sandalwood paste and breathed life into the idol. She then asked him to
stand on the gate and do not let any body enter until she came out.
Unfortunately, Shiva returned home in the meantime. As, Ganesha did
recognize him, he stopped Shiva from entering as per his mother's advice.
This badly enraged Lord Shiva, who cut off Ganesha's head by his trident.
When Parvati saw beheaded Ganesha, took on the form of the Goddess Kali and
threatened to destruct all the three worlds.

The earth, the heaven, the nether world, all was shaken and every body ran
to Shiva for solution. In order to appease Lord Parvati and save the world
from destruction, Lord Shiva sent out his followers to find a child whose
mother is facing another direction in negligence, cut off his head and bring
it quickly. The first such child that came in the eyes of the Shiva
followers was an elephant, so they brought the head of this elephant and
Shiva placed it on the trunk of Parvati's son and gave life into him.
Parvati was the overwhelmed with happiness and embraced her son. They named
her Ganesha i.e the Lord of all Ganas (followers).

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History
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Though the festival is being celebrated since times immemorial, the present
kind of celebrations of Ganesha Chaturthi came in fashion in 1893, Lokmanya
Tilak, an Indian freedom fighter and social reformer reshaped the annual
Ganesh festival from a private family celebration into a community event.
The day was conceived to be the National Festival in order to bridge the gap
between the Brahmins and the non-Brahmins in the society. Tilak chose this
festival for this purpose because Lord Ganesh was considered to be the ' God
of Everyman'. It then served as a meeting ground for people of all community
and religion on a public platform. Since then the festival has served its
cause of existence. Even now people irrespective of caste and community
barriers celebrate this festival with great joy.