Vinayagar + Chathurthi===>means Birth of Vinayagar.

Parvathi(Mother Goddess) the wife of Lord Shiva was thinking of taking bath but none of her attendants were there. So, she created a human figure from unguent and balm then with her power she gave life to the figure. She advised that human figure to guard the door while she was taking bath. She also advised that figure not to allow anyone to enter the home. The figure accepted the same. While Parvathi was taking bath, Lord Shiva (Husband of Parvathi) after a long Meditation on Mountain Kailash, came to home and he was astonished, since a boy was standing infront of his home. When Lord Shiva was about to enter the home, the boy stopped Him. Outraged by the act of this stranger, Shiva cut off his head from the body. Later, He came to know that the boy is Parvathi's Son. For fear of enraging his wife, Shiva ordered his ganas (attendants) to get him the head of the first living creature that they could find. But unfortunately, the Ganas saw an elephant first. Ganas as instructed, chopped the head of the elephant and brought it to Lord Shiva. Then he placed the same on Parvathi's head and gave him the life back. Once Parvathi took her bath, She came out of the home and she found a boy with an elephant head. She asked who is that boy (in tamil called as "pillai yaaru" means who is that boy"). From which Shiva named Parvathi's son as "Pillayar". This elephant-headed god was welcomed into the first family of the Hindu heavens and named Ganesha or Ganapathi, which literally means the chief of the ganas, or the attendants of Shiva. This brave guardian of the door to Parvathi's bath is beheld today as the most aspicious God of new beginnings.