Taking to Instagram Stories on Thursday, the Britain's Got Talent judge, 52, shared a picture of a candle she had lit in his memory.
She captioned the post: "You would have become a teenager today... #Theo," followed by a red love heart emoji.
Holden was 28 weeks into her pregnancy with music producer husband Chris Hughes when their son died and she had to deliver him by C-section.
Opening up about her loss in 2018 in an interview on Giovanna Fletcher’s Happy Mum Happy Baby podcast, Holden recalled the devastating moment that she was told her baby no longer had a heartbeat.
She said: 'They scanned my tummy... and it was one of the most surreal moments of my life.
"I remember hearing was this woman screaming and shouting and I was thinking, "Where is that coming from?" And it was coming from me and I didn’t even know I was doing it and I was doing it..."The obstetrician was trying to tell me that my baby's heart had stopped beating and was not alive anymore and I just was... saying, 'I don’t know what I’m going to tell Lexi,' and this is the woman I could hear, screaming that. And I didn’t know it was me. It was the oddest, most awful thing."More heartbreak was to follow when she learned that she would have to give birth to her son, just moments after learning he had passed away.
"The weirdest thing was, I forgot completely that I would have to give birth,' she recalled. 'That’s the other thing. You go, "Oh my god I have... get this baby out of me. And I absolutely cannot do that..." I don't how I did that…" she continued.
"The hours that followed were awful. I refused to have a natural birth, because I just thought I can’t do that, I need to have an operation and I need to get this baby out of me, and I was really cross and really heart broken and all the things.
"You just can’t imagine, I was absolutely scared to death and I kept saying to my midwives, "When he comes out, I don’t know if I am going to be able to hold him..." You know, I was so cross, scared to death.
"I remember Jackie [Holden's midwife] saying, 'We are really struggling to pull him out of you, he doesn’t want to leave you,' which was really sad.
"She held him in her arms and Chris left the room. He waited 'til right to the moment and said, 'Mandy, I can’t you know I need to leave the room.'
"And I respected anything he wanted to do, was totally fine by me, and he left the room and Jackie just described him to me and I just said, 'I just need him, I’m still a mummy.' It’s the weirdest sensation. And I just held him and he was perfect."
A year late, Holden - who also has daughters Alexa, known as "Lexi", 18, an Hollie, 12, with her husband - died for 40 seconds during the birth of her youngest offspring, before slipping into a four-day coma.
"I had a condition, placenta accreta. The baby was stuck to my bladder. I lost a lot of blood. It took 36 people to save me – and I can't thank them enough," she told The Guardian.
If you have been affected by this story, information and support can be found at Sands (The Stillbirth and Neonatal Death Charity). You can call them on 0808 164 3332 or email email@example.com