Theresa May, Donald Trump and 14 other world leaders will join the Queen and Prince Wales to commemorate the 75th anniversary of the D-Day landings in Portsmouth.

On 6 June 1944, Allied forces invaded Normandy to begin the liberation of Europe from Nazi and fascist occupation. D-Day is widely regarded as the biggest amphibious military invasion in history.

Commemorations start on Wednesday morning at Southsea Common in Portsmouth.

The event’s primary goal is to honour the Allied soldiers who risked and lost their lives during the operation. But the commemoration will also mark the end of Mr Trump’s state visit to the UK.

Here is a short guide to what you might want to know about the day.

D-Day 75: Normandy landings in pictures Show all 30 1 /30 D-Day 75: Normandy landings in pictures D-Day 75: Normandy landings in pictures Troops from the 48th Royal Marines land on Juno Beach, Normandy on 6 June 1944 Getty D-Day 75: Normandy landings in pictures A landing craft from the Coast Guard-manned USS Samuel Chase disembarks troops of the US Army's First Division on the morning of June 6, 1944 US Coastguard D-Day 75: Normandy landings in pictures An aerial view of the pierhead and breakwater at the artificial 'Mulberry Harbour' at Arromanches Royal Air Force/IWM D-Day 75: Normandy landings in pictures American troops wade towards the beach in the Normandy Landings Getty D-Day 75: Normandy landings in pictures A line of tank landing ships, each towing a barrage balloon, head towards the Normandy coast Getty D-Day 75: Normandy landings in pictures A flotilla of landing craft forms in the English Channel on the eve of the Normandy Landings PA D-Day 75: Normandy landings in pictures Crews of B-26 Marauders charged with laying smokescreens over beaches in advance of the Normandy Landings are briefed with their mission The LIFE Picture Collection/Getty D-Day 75: Normandy landings in pictures British soldiers joke as they read a tourist guide about France aboard a landing craft heading towards Normandy AFP/Getty D-Day 75: Normandy landings in pictures British boats arrive in Normandy PA D-Day 75: Normandy landings in pictures A view from above of Allied naval forces engaged in the Normandy Landings on 6 June 1944 AFP/Getty D-Day 75: Normandy landings in pictures American troops wade ashore on Omaha beach in Normandy on 6 June 1944 Getty D-Day 75: Normandy landings in pictures American medics administer first aid to wounded soldiers on Utah beach in Normandy, 6 June 1944 Getty D-Day 75: Normandy landings in pictures Canadian soldiers from 9th Brigade land with their bicycles at Juno Beach during the Normandy Landings on 6 June 1944 AFP/Getty D-Day 75: Normandy landings in pictures Consolidated B24L Liberators of the 832nd Bomb Squadron, 486th Bomb Group, US 8th Air Force, fly from their base at Sudbury, England to pass over the Normandy invasion fleet sailing in the channel Getty D-Day 75: Normandy landings in pictures A convoy of Allied landing craft, protected by barrage balloons, crosses the English Channel on its way to France during the Normandy Landings Getty D-Day 75: Normandy landings in pictures Ships towing barrage balloons land at Omaha Beach in the Normandy Landings Getty D-Day 75: Normandy landings in pictures Paratroopers give the thumbs-up signal, before leaving in a glider to drop on Normandy as reinforcements Getty D-Day 75: Normandy landings in pictures Royal Marine commandos move off the Normandy beaches during the advance inland from "Sword" beach PA D-Day 75: Normandy landings in pictures Royal Marine commandos move off the Normandy beaches during the advance inland from "Sword" beach PA D-Day 75: Normandy landings in pictures Allied paratroopers land on La Manche coast on 6 June 1944 after the Allied forces stormed the beaches earlier in the day AFP/Getty D-Day 75: Normandy landings in pictures US troops in landing craft, during the Normandy Landings Getty D-Day 75: Normandy landings in pictures Survivors from a landing craft which sank off Omaha Beach, Normandy reach the shore on 6 June 1944 Getty D-Day 75: Normandy landings in pictures American troops pass through devastated Valognes, on their way towards Cherbourg after the storming of Normandy, 13 June 1944 Getty D-Day 75: Normandy landings in pictures Members of the Allied Invasion Force keep watch from fox holes dug in a Normandy beach on 14 June 1944 Getty D-Day 75: Normandy landings in pictures Allied soldiers meet a herd of cows as they make their way through the Normandy country in June 1944, days after the Normandy Landings AFP/Getty D-Day 75: Normandy landings in pictures US soldiers surround a burning German tank in a Normandy village, days after the Normandy Landings AFP/Getty D-Day 75: Normandy landings in pictures US soldiers gather around trucks disembarking from landing crafts shortly after the Normandy Landings AFP/Getty D-Day 75: Normandy landings in pictures British troops during the Normandy Landings AFP/Getty D-Day 75: Normandy landings in pictures US troops at Cherbourg unload supplies for the advancing troops in the Normandy Landings Getty D-Day 75: Normandy landings in pictures A timetable of the Normandy Landings PA

So, what exactly is happening?

The UK has promised “unprecedented commemorations” and “one of the greatest British military spectacles in recent years”.

Over 4,000 military personnel will be involved in the UK and France.

The event will include testimony from veterans, theatre performances and live music.

In the morning, Ms May and the defence secretary, Penny Mordaunt, will be on the HMS Queen Elizabeth as they wave off around 300 veterans who fought 75 years ago. All of them are aged over 90 and will recreate the journey they made in 1944.

Two more veterans will fly across the Channel and parachute down over Normandy. One of them, Harry Read, is 95 years old.

The world leaders will then have a reception with veterans before sitting down to discuss the continued importance of the Western alliance and security, a Downing Street spokeswoman said.

Will there be a flypast?

Yes. The event will end with a flypast of 24 vintage and modern aircraft, including the Red Arrows and the historic Spitfire.

Which countries will attend?

Representatives from 15 countries – all of those who fought alongside the UK in Normandy – are planning to attend.

It will be the first time the UK has hosted so many world leaders outside a formal summit since the 2012 Olympics, the government said in a statement.

There will be presidents, prime ministers and other representatives of the United States, Canada, Australia, Belgium, the Czech Republic, Slovakia, Luxembourg, Denmark, France, Greece, the Netherlands, New Zealand, Norway and Poland.

German Chancellor Angela Merkel will also attend the commemorations in Portsmouth and have lunch with Ms May.

Any notable absences?

It doesn’t look likely.

So, what exactly were the D-Day landings?

On the night of 5 June 1944, Allied forces launched a naval, air and land assault on Normandy, a region in northwest France. D-Day was the first phase of Operation Overlord; the Allies’ plan to liberate Europe.

First, Allied forces bombarded the coast of France. Then, shortly after midnight, over 20,000 troopers parachuted into drop zones to establish a foothold in northern France. Later, at around 6.30 am, infantry and armoured troops began landing on the Normandy coast.

Nearly 160,000 troops crossed the Channel that day, and almost 900,000 landed by the end of June.

(https://www.statista.com/chartoftheday/Statista) (Statista)

Some 4,414 Allied soldiers were confirmed dead, but casualties are estimated to be over 10,000. Some 4,000 to 9,000 German soldiers are thought to have lost their lives.

The landings required unprecedented international cooperation and planning, including to stockpile resources and to mislead German intelligence about the exact date and location of the operation.

Germany expected an invasion and was planning to fortify the Atlantic coasts under its control, under a plan known as the "Atlantic Wall".

D-Day veteran makes first parachute jump since D-Day

What will Mrs May and Mr Trump say?

Ms May will describe the D-Day landings as a “moment of historic international cooperation” and emphasise that while the challenges the world faces today are different from those of 75 years ago, international cooperation and a Western alliance remain important today.

She will also read a letter written by Captain Norman Skinner of the Royal Army Service Corps to his wife Gladys on 3 June 1944, which was in his pocket when he landed on Sword Beach on 6 June. He was killed the following day, leaving his wife and two young daughters.

“I can imagine you in the garden having tea with Janey and Anne getting ready to put them to bed,” the letter reads.

“Although I would give anything to be back with you, I have not yet had any wish at all to back down from the job we have to do.”

We don’t know what Mr Trump will say. The commemoration will mark the end of his visit to the UK, which has been fraught with controversy and demonstrations.

What happens next?

The commemorations will carry on in Normandy on Thursday, where the military and French paratroopers will recreate the invasion.