This year 2018, Vinayagar Chathurthi falls on Thursday, September 13th.
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The festival of Ganesh Chaturthi finds its origin in the Maharashtra reign, with Chatrapati Shivaji starting the festival. The belief lays in the story of the birth of Ganesha, the son of Lord Shiva and Goddess Parvati. Although there are various stories attached to his birth, the one most relevant is shared here. Goddess Parvati was the creator of Ganpati. She, in the absence of Lord Shiva, used her sandalwood paste to create Ganesha and put him to guard while she had gone for bathing.
Shiva then ordered all his followers to immediately go and find a child whose mother has her back towards her child in negligence and bring his head. The first child seen by the followers was that of an elephant and they, as ordered, cut his head and brought to Lord Shiva. Lord Shiva immediately placed the head on Ganesha's body and brought it to life again. The rage of Maa Kali was calmed and Goddess Parvati was overwhelmed, once again. All the Lords blessed Ganesha and the day is celebrated for the same reason.
Celebration :-
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Ganesh Chaturthi preparations commence from almost a month before the festival. The celebration continues for five, seven, or ten days(from Bhadrapad Shudh Chaturthi to Ananta Chaturdashi). Some even stretch it to twenty one days, but ten is the most popularly celebrated. Prayers and cultural feasts are offered to Lord Ganesha every day during the festival.
A life-like clay model of Lord Ganesha is made 2-3 months prior to the day of Ganesh Chaturthi. The size of this idol may vary from 3/4th of an inch to over 25 feet. On the day of the festival, it is placed on raised platforms in homes or in elaborately decorated outdoor tents for people to view and pay their homage.
Homes are decorated with flowers. Temples witness the visit of large number of devotees. Poojas are performed and bhajans are chanted. Often, families gather together to celebrate the festival. Localities organize and arrange for pandals and install large idols of Lord Ganesha to celebrate the festival with friends and family. 
On the final day of the celebrations, the idol of Lord Ganesha is taken on the streets. People exhibit their enthusiasm and joy in the form of dancing and singing on the streets along with the idol. Ganesha is to be immersed in a river or the sea symbolizing a ritual see-off of the Lord in his journey towards his abode in Kailash while taking away with him the misfortunes of all man. The idol is finally immersed in the river or sea. The day witnesses a large number of devotees expressing their happiness and offering their prayers. 
Lord Vinayaga, the son of Shiva and Parvati, is believed to bestow his presence on earth for all his devotees in the duration of this festival. It is the day Lord Shiva declared his son Vinayaga as superior to all the gods. 
Vinayagar Chaturthi Pooja :-
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Vinayagar pooja starts from installing a clay idol of Lord Ganesha in your home. Various dishes are cooked for offering (bhog). The idol is given a bath with pure water and then decorated with flowers. Jyoti is lite and then the aarti begins. Various bhajans, and mantras are chanted at this time.
It is believed that chanting the mantras with complete devotion brings life to the idol. It is also believed that during this period, Ganesha visits the home of his devotees and bring prosperity and good fortune with him. For the same reason the day is observed as a very auspicious day.
Worshipping the Ganpati Yantra will bring you great success in life.
Content & Image Source : google
Disclaimer :- I have gathered all these information from family, friends and many Hindu websites. I do not make any representation or warranty with respect to accuracy, applicability or completeness.
Festival Dishes :-
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Although there are a large number of sweets offered to Lord Ganesha during the pujan, but Vella Kozhukattai (Sweet Modak) is known to be the lord’s favourite sweet and is therefore one of the main dishes made on this day.
Other dishes include Thengai Kozhukattai, Paal Payasam, Black Chickpeas Sundal, Peanut Sundal, Channa Sundal, Medhu Vadai, Moong dal Payasam, Rava Kesari, Puran Poli, Peanut Laddu, Sweet Aval.
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Cheers,
