Celebrities such as Rihanna are already sharing their excitement on social media ahead of tonight's Met Gala.

The charity ball is held at New York's Metropolitan Museum of Art every year on the first Monday in May.

It's an exclusive event, with tickets costing tens of thousands of dollars and a flamboyant dress code.

Newsbeat finds out what's said to make the so-called "party of the year" special.

What is it?

A black-tie event to raise money for the museum's fashion department, known as the Costume Institute.

Last year's gala raised approximately £10.5 million ($13.5 million).

It's also the launch party for the Met's latest big exhibition, which this year focuses on Rei Kawakubo, founder of fashion house Comme Des Garcons.

The Japanese designer is known for sculptural clothes that blur gender divides.

Guests are encouraged to dress to co-ordinate with the exhibition theme, and they normally make a huge effort, as pictures from previous Met Galas show.

So you can expect to see plenty of celebrities in huge space-age gowns and extravagant headpieces.

Who will be there?

This year it's being co-hosted by Katy Perry and Pharrell Williams.

And it'll be packed with the world's biggest celebrities and fashion designers.

Lady Gaga, Madonna, Gigi Hadid, the Kardashians and Beyonce have all been among the guests in recent years.

How much does a ticket cost?

A lot of money. Tickets this year cost around £22,200 ($30,000) each, and tables are about £212,850 ($275,000).

Lots of celebrities get in for free, as fashion brands invite them to sit at their tables.

In return, they're expected to wear clothes by that designer.

But they need to be high profile enough. There are only around 600 places, and everyone has to be approved by Vogue editor-in-chief Anna Wintour, who chairs the event.

There's already speculation about the celebrities she's denied access to this year's event.

What happens inside?

That's a closely guarded secret. We can expect to see plenty of behind the scenes action from celebrities on social media as they get ready.

We'll see them posing on the red carpet beforehand, and at the after party.

But for the past two years social media has been banned inside the event itself.

There will be a dinner, and a chance to see the exhibition, as well as music, which last year came from The Weeknd.

In a nutshell

It's an opportunity for the biggest stars to party in a stunning venue, many of them for free.

The exclusive guest list no doubt adds to the appeal.

But most of all it's about the fashion. This is essentially the world's most luxurious fancy dress party, costumed by the top designers. Who doesn't love a chance to dress up?

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