What's that saying? The one parents use when they're talking about their kids, or grandparents about their grandkids. "They grow up so fast." No, that doesn't sound quite right. Oh yeah: "They monopolise the market so fast." That's the one. 

OK, fine. No one says that about their children. But it's what we imagine the proud parents of the once babyfaced Five Guys chain are saying now as the brand prepares to open its 100th store. 

The new restaurant in St Paul's will open its doors on Monday 9 December, which just so happens to be another landmark date for the burger restaurant: the day it will finally launch its long-anticipated breakfast menu.

To celebrate the double whammy of occasions, the new location is offering an entire day of freebies. As in, everyone is entitled to free breakfast and free lunch or dinner – depending on which time you'd rather inhale one of their famous patty and buns. The deal includes an orange juice in the morning, as well as a soft drink and fries in the afternoon or evening. 

The first 500 people to visit the new branch on the day will also be offered some exclusive Five Guys clobber as well, including branded hats, umbrellas and jackets. If you're going to Glastonbury's 50th anniversary in the summer – or indeed any other musical gathering – let's not pretend a signature red and white Five Guys bucket hat wouldn't look cool. Because it would. It just would.

The diner-style restaurant has earnt a loyal following of superfans, with everyone from Adele to Barack Obama regularly endorsing it

Five Guys has a reputation for serving food that bypasses many of the greasy undertones associated with the more traditional drunk-food, or hangover-feeding fast food spots.

Since opening its first store in Covent Garden in 2013, the American diner-style restaurant has gone on to earn a loyal following of superfans, with everyone from Adele to Barack Obama regularly endorsing the brand for its tasty dishes.

John Eckbert, CEO of Five Guys Europe, said: “We have been overwhelmed by the support Five Guys has received from the British public over the last six years. From being named UK’s favourite fast-food chain for two consecutive years, to becoming Deliveroo’s most ordered dish – we have lots to thank our fans for.

"We couldn't see a better way to do this than to invite them to share a freshly made burger with us at our 100th store.”

As it happens, we can't think of a better way to be thanked. Thank you right back, Mr Eckbert.

The new Five Guys will open 9 December at 9am. For more information, visit fiveguys.co.uk