Why is the annual festival Diwali an important one for the Hindus?

Deepavali greeting. abplive.in/
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Among the colorful   and fascinating  festivals of India, the most popular one is Diwali or Deepavali, the festival of light, which symbolizes inherent brightness of India's soul and ethos; here light is a  an allegory of  knowledge and consciousness. and of soul and spiritual energy.   The word Diwali has its origin in the Sanskrit word ''deep'', (in Tamil Deepam/Vilakku) meaning an Indian lantern/lamp. In the  unique social landscape of  India no other festival brings out the spirit and  fusion of different cultures and national integration  better than Diwali.
Diwali festival. lighting in a Hindu temple.  deccanchronicle.com
As we are aware,  Diwali  marks  the spiritual "victory of light over darkness, the  good over  the evil and knowledge over ignorance."  Here, ''the victory over the evil''  refers to the Hindu mythological lore where-in  Lord Krishna's Victory over Narakasura, the demon ruler who troubled people of all walks of life.  This festival also commemorates the  return of lord Rama, an incarnation God Vishnu  to his home Ayodhya after fourteen years of  long Exile from his kingdom and his  defeating  of demon king Ravana and freeing Sita, Rama's consort. .

Though it is a one day festival, preparations begin a few days before the main ceremony. They include purchasing of dress, jewelry, etc for the family members, buying or preparing  mouth-watering sweets, savories, etc. ''Dhanteras'' refers to  annual renewal, cleansing and an auspicious start of the next year. It marks the thirteenth day of the dark fortnight of Kartik and the beginning of Diwali and festivities begin  on this day and will will go on in the next few days or more. It many states it is a 5-day celebration as follows:
01. First days is Dhanteras or the day of  wealth and fortune.
02. Second day is Naraka chaturdasi or the days of knowledge.
03. Third day is Diwali or the day of light.   
04. Fourth day is Annakut or the beginning of the new year for    Hindus.
04. Fifth day is Bhai Duj or the day of  sharing affection among siblings.

Taking non-vegetarian food during festive  days is  a taboo among Hindus  and is fully avoided during Diwali. The newly married couples  normally get priority  and  receive special gifts from their parents and in-laws as they will be celebrating this festival for the first time after their wedding. The tradition of giving special gifts to the newly weds has been around for  centuries. The festival day coincides with the darkest night of the Hindu lular month ''Kartika''. In the Gregorian calendar, the festival generally falls between mid-October and mid-November.
 In many states, the tradition has been that  people get up in the early morning and take a sort of ritual  oil bath (applying gingely oil all over the body; normally it is done by elders)  before sunrise, put on new dress and light fire-crackers. Particularly for kids and children, it is the most joyous occasion to savor sweets and light fire-crackers. In some states, Hindus worship Goddess Durga (Kali), especially  in East India, in the south God Sri Krishna and in the western states Goddess Lakshmi  (Goddess of wealth). In the case of families connected with  business or trade. 'Lakshmi puja' is quite popular and many  of  them will open a new business account  afresh, invoking Goddess' blessing on this day.  Normally, in all homes,  at dusk  rows  of oil  lamps are lit up  and kept in the front part of the house or inside to welcome  Goddess Lakshmi and to get rid of negative forces. Unlike other festivals, Hindus do not observe  fasting  during this festive event, rather  they feast and share the food, etc with others.  In many  community centers, temples etc poor people are offered free food. Sharing food  with the  poor and needy is part of this festival. Countless people send donations to orphanages that take care of the orphans and others.
Diwali  festive mithai (sweets),en.wikipedia.
During the festival day(s), temples, homes, shops and office buildings are brightly lit up and devout Hindus will never fail to visit the temple in the evening, invoking god to bless  them with welfare and good health in the ensuing year.

 www. sharingourexperiences.com/why-do-we-celebrate-diwali-in-ndia-how-to-explain-the-significance-of-diwali-festivals-of-light-to-your-child/#why-do-we-celebrate-diwali-festival https://en.wikipedia.org/wiki/Diwali