Diwali (Deepavali)
It is called as the Festival of Light and comes in the month of Kartika (Oct/Nov).
Many mythological stories prevail for Diwali, the major two being the return of Lord Rama to Ayodhya after killing Ravana and the second being the Death of Titan of Hell, Narakasura by Lord Krishna. Both the events signify winning of good over evil, people light candles, diyas in their houses, worship Laxmi (Goddess of Wealth) and visit near dear ones, distribute sweets and wear new clothes etc.

Holi
Also called as Festival of Colors, it is celebrated in the month of March.
The mythological story behind Holi is of Prahlada, Holika and Hiranayakshyup which signifies Holika buring to ashes and Prahlada emerging as great worshipper of Lord Vishnu and defeater of demon Hiranayakshyup. People celebrate Holi by throwing colored water and bright powders on each other and enjoying themselves.

Onam
Celebrated in Kerala to mark the end of summer monsoons.
It has a mythological story of welcoming demon King Mahabali to his lost kingdom.

Raksha Bandhan
Celebrated in the months of July/August.
A sister ties a Rakhi [sacred thread] to the wrist of her brother/s who in turn give her some presents and become responsible for her safety.

Maha shivaratri Day dedicated to Shiva, god of destruction and remover of injustice, ignorance and all things evil. Observed by a day-long fast and a night-long vigil.

Rama Navami
Celebrated as the Birthday of Lord Rama.

Ganesh Chathurthi
Celebrated as the Birthday of Lord Ganesha.

Navratri
Festival of Nights, Lasts for 9 days with 3 days each devoted to worship of Goddess Durga (Goddess of Valor), Lakshmi (Goddess of Wealth) and Saraswati (Goddess of Knowledge).

Saraswati Puja
A Festival honoring Goddess of Knowledge.

Shivaratri
Celebrated as the Birthday of Lord Shiva.

Krishna Janamashtami
Celebrated as the Birthday of Lord Krishna.

Nori-Nem
Nori-Nem is a type of 'Vrat' (a religious fasting to keep a gaol in mind) and in the spiritual world, is done to apply a control over ones pleasures. 'Nori-Nem' carries a great significance and is observed during the month of 'Sravana' (worshiping of Lord Shiva). The 'Vrat' is observed mainly by ladies, who make a murti of a mongoose from 'juvar' (corn flour) and 'haldi' (tumeric) and place it on to a 'baajat' (square stool) to do a puja. The Ingredients for the puja include mung beans, juvar, baajari and unhusked grains, which are offered to the mongoose. These ingredients are then given over to a priest as charity is an important element of this 'Vrat'. Finally, the lady also includes in her meal the mung beans and grams.

'Nori-Nem' stems from a story in which a poor Brahmin's wife, who was a mother full of faith and devotion, loved her child more than her own life. One day, her infant child was sleeping in the cot. The Brahmin had gone to the temple and the mother decided to go to the village well to fetch some water for the house. Just then, a black snake slowly descended from the roof of the house and climbed onto the cot. The child then awoke with an innocent smile on the face. The snake kept staring at the infant but then raised its hood in preparation for striking. Within a fraction of a second, before the fangs of the snake could deliver its deadly poison to the infant's feet, a mongoose caught hold of the snake and removed it from the cot. Just then, the infant's mother returned to the house and saw the mongoose and the snake fighting. She then realised that her devotion of performing a 'Vrat' had been rewarded by a 'Devata' (a devine sprit or being) coming as a mongoose to save her child.