There are many religious festivals which change dates every year.

This month Hindus across the world will soon celebrating the festival of Maha Shivaratri.

It's a key event in the religious calendar but it has a different date every year.

In 2017, the date of the Maha Shivaratri was Friday, February 24.

So when will it be this year? And how is it celebrated?

What is Maha Shivaratri and what does it celebrate?

A woman dressed as Hindu goddess Kali participates in a procession on the eve of Maha Shivaratri in Jammu, India, in 2014.

Shivaratri - sometimes written Shivratri - means Night of Shiva.

This is a day in honour of the god Shiva and is held on the 13th or 14th day of every month.

But once a year, there is a Maha Shivaratri - or Great Night of Shiva - on the new moon day in Maagha, the 11th month in the Hindu calendar.

This usually falls in February or March, before the arrival of spring.

In 2018, the date of the Maha Shivaratri is Tuesday, February 13.

What are the origins of this festival?

It was on this day that Shiva is said to have saved the world from destruction on condition that people worshipped him with great enthusiasm.

Another legend says that Shiva simply named this as his favourite day when asked by the goddess Parvati.

A third story says that a hunter was unable to find any game and so decided to wait in a woodapple tree. He threw down leaves in the hope they would attract deer but was unaware there was a sacred worshipping place to Shiva beneath the tree.

The god himself appeared and blessed the hunter with wisdom for his patience. From that day, the man stopped eating meat.

Religious texts explain: "During the Vigil Night of Shiva, we are brought to the moment of interval between destruction and regeneration; it symbolises the night when we must contemplate that which watches the growth out of the decay.

"During Maha Shivaratri we have to be alone with our sword, the Shiva in us. We have to look behind and before, to see what evil needs eradicating from our heart, what growth of virtue we need to encourage.

"Shiva is not only outside of us but within us. To unite ourselves with the One Self is to recognize the Shiva in us."

So what happens at Maha Shivaratri?

An Indian woman dressed as Hindu goddess Kali participates in a Shivaratri procession in Allahabad, India, in February 2014.

This is a major Hindu festival about overcoming darkness and ignorance in life and the world.

Unlike most other festivals in Hinduism, the Maha Shivaratri is held at night and is a solemn occasion rather than a time of revelry.

The focus is on fasting , prayers, poems, meditation and reflection and also includes an all-night vigil at temples, which are adorned with lights and colourful decorations.

This is a time to focus on restraint, forgiveness, repentance, honesty and kindness.

Offerings are made to Shiva of woodapple leaves, fruit, cold water, flowers, sugarcane, sweets and milk, while some worshippers fast all day and night and others do meditative yoga.

Fasting is believed to bring good luck to those who do it.

In temples dedicated to the deity, a sacred mantra is chanted.