Ron Jones, from Bassaleg, in Newport, only retired from collecting for the Royal British Legion Poppy Appeal in 2016, after 30 years' service.
Mr Jones's advice for living to a ripe old age is to simply "live your natural life, that's all I've ever done".
He is celebrating his centenary with a family party.
Mr Jones was captured in the Middle East during World War Two and said he arrived at Auschwitz in October 1943.
He said he never spoke about his experience at Auschwitz until the release of a Steven Spielberg film about the holocaust in 1994.
"Nobody seemed interested until they put the Schindler's List on and then everybody found out about it of course," he said.
Speaking about his arrival at the camp, Mr Jones told BBC Wales: "We saw all this barbed wire and men in pyjamas digging trenches, and someone said to one of the guards 'who the heck are they?'
"He said 'Jews', just like that, as though we should have known.
"And we couldn't understand what the smell was. Someone asked, and he said 'oh that's the crematorium'.
"It took us two or three weeks before we actually accepted that they were gassing and burning them."
He has previously recounted his experiences at the camp where he worked six days a week in a factory and being permitted to play football with his fellow soldiers on Sunday.
Mr Jones said he used playing football with a "rag ball" as a form of escapism and embroidered the Prince of Wales' feathers on his shirts using old socks.
"It was a relief, you know, to do something."
But he said he returned home a different person: "It altered me. My wife used to look at me and say 'you're not the man I married'.
"Well, I wasn't. I was a changed man completely when I came home. I was in a hell of a state.
"I used to get nightmares and boils and all sorts.
"It took me about four or five years before I was a normal man again, and then she used to kiss me and say 'you're back to normal'."
Of his 100th birthday celebration, Mr Jones quipped: "I'll be glad when it's over because of the fuss they're making."
Ant Metcalfe, the Royal British Legion's manager for Wales, said: "By sharing his incredible story of survival and resilience with the press and media Ron has touched so many people and left a lasting impression on many."
Mike Jones, Poppy Appeal organiser for Gwent, added: "Ron is a true ambassador, such a character and a real gentleman.  Not only [was] he one of our oldest collectors but one of our most popular."

The English pairing of Judd Trump and Barry Hawkins topped Group C and come up against Mark Allen and Joe Swail, runners-up in Group D, in Wuxi.
Group A winners Wales, with Mark Williams and Ryan Day, play Group B runners-up China A, who have Ding Junhui and Liang Wenbo in their team.
Holders China B against Belgium and Thailand v Iran complete the line-up.
At the previous event in 2015, teenage Chinese duo Zhou Yuelong and Yan Bingtao, now 17 and 19 respectively, shocked Scotland's former world champion John Higgins and Stephen Maguire to claim the title.
This year, the Scots, who had Anthony McGill in place of Maguire, failed to reach the knockout stages after finishing third in Group D.

Dasilva, 19, is yet to play for Chelsea but impressed in 10 appearances for the Addicks at the end of last season.
The full-back captained England Under-19s to their first win at the European Championship this summer, as the Young Lions beat Portugal 2-1 in the final.
"He brings pace, talent, desire and a willingness to improve," Charlton boss Karl Robinson told the club website.
Find all the latest football transfers on our dedicated page.
One of Britain's oldest poppy sellers, who was a prisoner of war in the Nazi death camp Auschwitz, celebrates his 100th birthday on Sunday.
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Jay Dasilva has signed a new four-year deal with Chelsea and rejoined Charlton Athletic on a season-long loan.