All Saints Day, 1 November, is when Catholics remember the saints and their own deceased family and friends.
In Poland, where it is a public holiday, people light candles at their relatives' graves.
Gunnersbury Cemetery in west London contains about 1,600 graves belonging to Poles or people of Polish origin, including many war veterans.
Volunteers from the Poland Street association are in attendance; about 100 people have turned up to sweep away leaves and light candles contained in glass lanterns, or znicze.
"This beautiful way of remembering those who have passed away takes on a new symbolic meaning abroad," says the association's spokeswoman, Anna Galandzij.
"It brings together older and younger generations of Polish expats, which is much needed now the UK has voted to leave the EU."
She adds: "The UK has been home to many generations of Poles, including the Polish Government in Exile, pilots who fought in the Battle of Britain, and soldiers of the Polish Home Army, who couldn't go back to Poland after the Second World War.
"Every candle we light on their graves is a tribute to what they did."
The president of the Polish Government in Exile, Kazimierz Sabbat, is among those buried at Gunnersbury and his daughters are present to see the clean-up.
Jolanta Sabbat and Anna Swidlicka say they are moved by the actions of Poland Street volunteers and the Polish Scouts who have given up their time.
"Attending to the graves reminds us of our roots," Anna says.
When her father understood that Communism would last in Poland and could not go back because he had served with Polish troops based in Scotland, "he taught us to respect the country we were in but also to remember our Polish heritage".
Elsewhere in the cemetery, friends Antonia Kocicowa and Zbigniew Konicki are lighting candles at the Katyn Memorial, commemorating the massacre of 22,000 Polish prisoners of war by the Soviet Secret Police in 1940.
On the other side of London, at St Patrick's Catholic Cemetery in Leytonstone, Agnieszka Adamska is co-ordinating more volunteers as they tend to about 300 Polish graves.
"It warms my heart when so many of us cultivate Polish traditions," she says.
"The act of getting together to remember our ancestors has a much deeper meaning than taking part in Halloween celebrations."
Meanwhile, Marek Stella-Sawicki, chairman of the Polish Heritage Society, has been coming to South Ealing Cemetery for 47 years to attend the graves of the many, largely forgotten, Polish servicemen laid to rest there.
He is keen to highlight one grave in particular, that of Stanisław Jachnik, the colonel of the Polish Airborne Brigade.
It was involved in the Battle of Arnhem in September 1944, when Allied troops were caught in a disastrous operation behind Nazi lines in the Netherlands.
Marek describes how Colonel Jachnik was among those parachuted into the town of Driel in September 1944.
"He was critical in helping the First British Airborne withdraw from Oosterbeek on the northern coast of the River Rhine. It is our duty to remember these people who risked their lives," he says.
Amid the revelry and dressing up at Halloween comes a more solemn occasion - one that is being upheld by the UK's Polish community this week.