Most of the world's 1.8 billion Muslims – roughly a quarter of the Earth's people – have begun observing the rites of Ramadan, which will last from this week's new (crescent) moon until the next new moon, 29 or 30 days from now. This includes Australia's 600,000 Muslims, according to the last census, with backgrounds ranging from Middle Eastern to south-east Asian.

Here's what they're likely to be doing – and not doing.

What is Ramadan and what is it for?

Ramadan is a kind of "spiritual training ground", says Islamic Council of Victoria president Adel Salman, the "main game" of which is to improve your relationship with God and improve yourself.

It may go for just a month but the idea is to develop new and better habits long-term, Salman explains. "It's a bit like New Year's resolutions," he says.

Ramadan is the Arabic word for the ninth month in the Islamic calendar, sometimes rendered as Ramazan in other languages. As the Islamic calendar is lunar (organised by the cycles of the moon) and the Gregorian calendar used by most Western countries is solar, Ramadan's timing in the Gregorian year shifts by roughly 11 days each year. This year, Ramadan falls across the Gregorian months from early May to early June. Last year, it fell 11 days later – from mid-May to mid-June.

The timing is much discussed among Muslims, says Salman, with some Muslims simply looking to the Bureau of Meteorology or phone apps for advice on the new moon while traditionalists prefer to wait for an authority to actually see the new moon in the sky the night before it becomes "new".

In any case, the month has a special status because Muslims are taught that the Koran – which they revere as the word of God – was revealed to the Prophet Muhammad during this month, beginning in the year 610. During this month, Muslims are expected to undertake prayer and abstinence in thanks for God's revelation and his creation, as a result of which they will emerge spiritually purified.

The Koran tells Muslims that on the holiest night, Lailat al-Qadr (The Night of Power), which falls during the last 10 days of Ramadan and marks the beginning of God's revelation to Muhammad, any prayer they make will be better than the prayers of 1000 months and that angels descend to Earth to carry out God's wishes.

Fasting during Ramadan is one of the five pillars of Islam, along with the testament of faith (a verbal declaration of adherence to Islam), regular prayer, charitable giving and the Haj pilgrimage to Mecca.

Who does it?

Every Muslim who was reached the age of responsibility (puberty) and is mentally and physically capable is expected to observe Ramadan.

What do they have to not do?

Between sunrise and sunset, Muslims have to abstain from eating, drinking, ingesting medications, smoking (which is generally frowned upon in Islam) and sexual intercourse. A single sip of water or coffee, or a puff of a cigarette, is enough to invalidate the fast. This is as true in Australia as anywhere.

To prepare for their daily fast, they wake for a pre-dawn meal called suhoor: vegetables and fruits, tea, yoghurt, dates, and foods that offer lasting sustenance such as beans and lentils. In many cities in the Muslim world, volunteers wake the faithful for suhoor by marching through the streets chanting and beating drums.

Many Muslim-majority countries curb the sale of alcohol during the month. In some countries, people who eat in public during the day can be fined or even jailed, although adherence to Ramadan etiquette by non-Muslims is often a personal choice and not enforced by police.

In the United Arab Emirates, which has large Western expatriate populations in Dubai and Abu Dhabi, restaurants use curtains to conceal customers who eat during the day. In Saudi Arabia, restaurants simply close until evening.

The fasting acts as a reset for the mind, body and soul. Ramadan is also a month of gratitude. By abstaining from food and water during the day, the faithful are reminded of those less fortunate.

Muslims are also expected to refrain from misdeeds, idleness and hateful speech.

What do they do?

Once the start of the holy month is declared, Muslims share holiday greetings such as Ramadan Mubarak, or Blessed Ramadan, via text messages, calls and emails to family and friends.

Ramadan is a month of worship. Muslims are expected to show self-control and deeper spirituality, spending time in contemplation and prayer – although they can also go to work.

Muslims pray during the day all year but during Ramadan there is another ritual: taraweeh prayers are observed after the evening meal, or iftar, either at home or in congregations at mosques.

Muslims can shop for food during the day but just can't eat it. At sunset, they traditionally break their fast, as the Prophet Muhammad did some 1400 years ago, with a sip of water and some dates. After sunset prayers, a large feast is shared with family and friends.

Iftar is a social event as much as it is a gastronomical adventure. Across the Arab world, apricot juice is an iftar staple. In South Asia and Turkey, yoghurt- based drinks are popular.

Mosques and charitable organisations will open their doors to feed the poor and needy – particular merit is attached to performing charitable works during the month. In Australia, this can include volunteering at the local mosque, helping with aid groups or donating to charities, says Salman. "Even a smile to someone is considered an act of charity."

Don't people get hungry and tired?

Yes. In some countries where Islam is the chief religion, the working day is shortened for Ramadan. It's particularly onerous when it falls during the summer months. Muslims living in countries with excessively long daylight hours are advised by religious scholars to adhere to the fasting times of the nearest Muslim-majority country.

However, just as Christian holy seasons such as Christmas and Easter have become commercialised, Ramadan is increasingly associated with night-time festivities and binge eating. While, traditionally, the fasting day ends with a feast, in modern times people often attend Ramadan events at hotels and restaurants and, combined with the lower activity of fasting days, can even find themselves gaining weight during the holy month.

In the Gulf states, a spike in attendances at hospitals has been reported, with problems ranging from dehydration to uncontrolled diabetes, as well as injuries from traffic accidents attributed to drowsiness.

Can you "apply for an exemption"?

Children, the infirm, pregnant or breastfeeding women and those with chronic health conditions are all understood to be exempt from fasting. Women who are menstruating or people who are ill or travelling on long journeys can also fast at alternative times of the year.

The Koran makes it clear that the point of fasting is self-discipline and not to put oneself through unnecessary suffering and hardship. "Allah intends for you ease and does not intend for you hardship," it says. As a sign of his piety, the Prophet is said to have fasted two days in every week – something television journalist Michael Mosley cited when he took up the 5:2 diet.

In recent years, questions have been raised over the assessment of students and examinations during the fasting month. “They struggle to concentrate and their energy levels are low,” said a school principal.

What else happens during Ramadan?

One particular modern phenomenon is month-long TV series. In the Middle East, in particular, people who are less active because of fasting mean a captive audience for broadcasters, who will schedule soap operas or historical dramas to run in the evening throughout Ramadan. Islamic authorities have criticised these shows for distracting people from the holy month's primary purpose.

How does Ramadan end?

As discussed above, traditionally Muslims rely on an authority to sight the new moon to determine the beginning and the end of Ramadan.

For example, some Sunni Muslims wait for the Saudi Arabian religious authorities to declare the end of Ramadan. In Nigeria, where more than 80 million Muslims live, the Sultan of Sokoto – a hereditary religious authority – formally asks Nigeria's Muslims to look for the moon so that the beginning and end of the holy month can be determined.

Muslims mark the end of Ramadan with a celebration and feasting known as Eid al-Fitr, the Festival of Breaking the Fast. In Indonesia this festival is the most important one in the calendar and is known as Lebaran. In Australia, there is a lot of buzz, including giving presents to children, visiting family and friends, and eating lots of special sweets and pastries. Women dress in colourful hijabs and men spruce up too. "It's quite a celebration," says Salman.

– with AAP, The Washington Post, Felicity Lewis