Millions across India are set to celebrate Diwali, the Hindu festival of lights, illuminating homes, temples, offices and streets with oil lamps and colourful lights to symbolise the triumph of light over darkness.

The festival falls on the darkest night of the Hindu lunar month of Kartik, when the moon is not visible in the sky.

Although Diwali traditions vary across India's regions and communities, the festival is widely marked by prayers, family gatherings, the exchange of sweets and the lighting of firecrackers.

But the celebrations come amid growing concerns over air pollution, particularly in northern India.

Seasonal weather conditions and the burning of crop residue contribute to deteriorating air quality, while firecrackers set off during Diwali can make the situation worse.

In the capital, Delhi, the air quality index (AQI) reached 391 on Friday morning, placing it in the "very poor" category ahead of the festival. There are fears that pollution levels could rise further after the celebrations.

India's financial capital, Mumbai, has also experienced high pollution levels in recent weeks, with its AQI reaching 322 on 24 October. Although air quality has improved in recent days to reach "moderate" levels of 104 on Friday, there are concerns that Diwali fireworks could trigger another spike.

In an effort to limit pollution, India's Supreme Court on Monday permitted the manufacture, sale and use of certified "green" firecrackers in Delhi and surrounding areas for a limited period.

These firecrackers are designed to produce 20-30% less emissions than conventional fireworks. But critics have questioned how effective they are in reducing pollution and raised concerns that conventional firecrackers could still make their way into the capital from neighbouring states.

Despite the debate over fireworks, Diwali is about much more than firecrackers.

The festival celebrates the return of Lord Ram to Ayodhya after his victory over over Ravan, the main antagonist in the Hindu epic Ramayana, and symbolises the triumph of good over evil.

In the days leading up to the festival, families clean and decorate their homes with flowers, colourful lights and traditional oil lamps known as diyas.

The celebrations also bring families and friends together for prayers, elaborate feasts and the exchange of gifts, and lots of traditional Indian sweets.

Colourful patterns known as rangoli are created at entrances to homes, while markets bustle with shoppers buying festive essentials beneath brightly lit decorations.

The celebrations began on 6 November with Dhanteras, when buying gold, silver or new utensils is considered auspicious and believed to bring good fortune.

The second day, known as Chhoti Diwali, or small Diwali, is marked by the lighting of oil lamps and, in some households, setting off firecrackers.

The third day is the main celebration, when families offer evening prayers to Lakshmi, the Hindu goddess of wealth and prosperity, before gathering for festive meals and fireworks.

The fourth day is celebrated differently across India, with family gatherings and New Year festivities in some communities. The fifth and final day, known as Bhai Dooj, celebrates the bond between brothers and sisters.

The festival is also celebrated by Sikhs and Jains along with other important religious observances.

Sikhs mark Bandi Chhor Divas, or Prisoner Release Day, commemorating the release of their sixth guru, Guru Hargobind, from imprisonment at Gwalior Fort in the early 17th Century.

For Jains, Diwali marks the attainment of moksha, or liberation from the cycle of rebirth, by Lord Mahavira, the 24th and last Tirthankara, or spiritual teacher, of the current era.

With Indian communities spread across the world, Diwali is also celebrated internationally, with families and friends coming together for festive gatherings and exchanging gifts.

The festival is considered an auspicious time to make new purchases, which many believe bring prosperity and good fortune. India's stock exchanges even hold a special one-hour trading session for investors who consider buying and selling shares on Diwali auspicious. Known as known as Muhurat trading, it will take place on 8 November this year.

Although rooted in Hindu traditions, Diwali is celebrated by people across different faiths and communities.