Ultra-Orthodox Jewish men pray along the Mediterranean Sea in the Israeli city of Herzliya (Picture: Getty Images)

Today is one of the holiest days in the Jewish calendar – Yom Kippur.

It's the Day of Atonement meaning a time to ask forgiveness for the sins of the past year, as well as looking at the year ahead.

Millions will be marking the date with 25 hours of fasting as well as prayers and spending time with family and friends.

For those who want to add a special touch to the day with a greeting in Hebrew, here are a number of things to say.

How to greet people during Yom Kippur in Hebrew

Ultra-orthodox Jewish man stand in a praying books stand on Yom Kippur eve (Picture: Getty Images)

The best greeting to give someone during Yom Kippur is 'G'mar Fatima Tova'. This means 'may you be sealed in the Book of Life'. You can also greet someone with a shorter version of this phrase by saying 'G'mar Tov'.

You can also say 'G'mar Hatimah Tovah', which means 'may you be sealed for a good year [in the Book of Life]', or 'Tzom Kal' which means 'have an easy fast'.

Some Rosh Hashanah greetings can also be used for Yom Kippur as they are both part of the Ten Days of Awe.

These include 'L'Shana Tovah' which means 'for a good year' and 'Chag Samayach' which means 'happy holidays'.

Yom Kippur: Everything you need to know

When does Yom Kippur usually fall?

An ultra-Orthodox Jewish man swings a chicken, later to be slaughtered as part of the Kaparot ritual in which it is believed that one transfers one's sins from the past year into the chicken (Picture: ABACAUSA)

Yom Kippur officially starts at sundown tonight.

The date does change from year to year on the gregorian calendar but typically takes place in either September or October.

On the Jewish calendar Yom Kippur is always the 10th day after Rosh Hashanah – the Jewish New Year.

How do people mark Yom Kippur?

Members of an Orthodox Jewish community study the Torah in Williamsburg, Brooklyn on Yom Kippur (Picture: Getty Images)

Yom Kippur is all about fasting but people also abstain from bathing, wearing perfumes, lotions and wearing leather. That's why you'll see plenty of smartly dressed people on the way to synagogue wearing trainers. Many people also wear white for the day.

Most Jewish people will spend the day in synagogue. Many secular Jews, who don't otherwise go to synagogue will attend for at least this one day.

The idea is that the day will be spent in prayer, both publicly and privately, to ask forgiveness for one's sins. By the end of the day Jews hope that they've been forgiven for their sins of the past year and can start afresh with a clean slate.

All in all there are a grand total of five special prayer services that take place during Yom Kippur.

While Yom Kippur is a serious day – the most solemn day of the Jewish year – there is an undertone of joy too. This is the confidence that God will forgive sins and the next year will be full of health and happiness.

Why do people not drink water?

An ultra-Orthodox Jewish man lightly whips a Jewish man inside a 'yeshiva,' or religious seminary, in Jerusalem prior to the start of the high Jewish holiday of Yom Kippur (Picture: ABACAUSA)

Really anyone observing Yom Kippur should avoid any food or drink at all for 25 hours. Yep, even water.

The night before the festival starts Jewish families come together to eat a pre-fast meal before lighting candles, saying a blessing to the children and going to synagogue for a special service called Kol Nidrei.

Is there a pretty good feast afterwards?

An ultra-orthodox Jewish man performs the Kaparot ceremony  (Picture: Getty Images)

Yom Kippur is closed with the Neilah service, after which there is sometimes song and dance, and a fast-breaking meal.

What people break the fast with tends to depend on individual families.

Some people start with a glass of water and something small like a slice of honey cake, others prefer to have a large meal – it really comes down to personal taste.

How to children mark Yom Kippur?

Ultra-orthodox Jewish man stand in a praying books stand on Yom Kippur eve (Picture: Getty Images)

Fasting only starts once children have turned bar mitzvah/bat mitzvah – which means they're 13 (sometimes 12 in the case of girls, depending on the branch of Judaism).

However, many younger children mark the day in some way, such as missing a meal, or avoiding treats.

Mandy Mazliah is a vegetarian mum of three, whose kids hate vegetables! She develops recipes and blogs about her efforts to get her kids to eat more veg at Sneaky Veg.

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