It was when their son Aaron first fell ill, and when they heard his brain tumour had returned. Three years ago this week, it was also when he died, at the age of 24.
The pain is still raw, say his parents Crystal and Richard Wood. But, as they front a Christmas campaign to find a cure, it is also what that fuels them.
So that another family, facing such a devastating diagnosis, might one day escape the pain they've endured.
"You never think it will happen to you, until it does," said Mrs Wood, 47. "The loss of a child. And you wonder how a person, a family, can cope with such pain.
"Aaron is missed beyond words. We keep going because that's what he would want us to do. He was all about living life. That's all he wanted."
Diagnosis and treatment
Aaron, brought up in Rotherham and a former student at Brinsworth Academy and Catcliffe Primary, had been home from university for Christmas when he had his first seizure in 2012.
There was a trip to hospital, CT scans, an MRI. He had a low grade brain tumour called an oligodendroglioma, doctors said, but it was 'slow growing'.
They agreed to delay surgery, so he could finish his philosophy studies at the University of Essex. It wasn't to be. At Easter, they found the tumour had grown.
A week later, he asked to come home. He had suffered another seizure.
The surgery removed at least 90 per cent of the tumour. But then came a further blow, it had progressed to stage 3, reclassified as an anaplastic astrocytoma.
On the day he should have graduated university, in July 2013, he started intensive radiotherapy.
"Driving through Sheffield city centre on the way to the hospital, we could see all the students celebrating," said Mrs Wood.
"It was really hard. He just looked at me and said 'I'll go back next year, and get it'."
Aaron was to return, and graduated with a high 2:1 the following year, but the treatment had taken its toll.
At Christmas 2015, the family found out the tumour had returned. By September, despite chemotherapy, it was clear it couldn't be cured.
"There was nothing more they could offer him," said Mrs Wood. "He didn't want a time frame, he said he wanted to live every day as best he could.
"The day we got his terminal diagnosis, Aaron put his hand on my shoulder and said 'mum, it's OK, we have an answer'. It wasn't the answer we wanted, but it was an answer.
"He was exceptional. We found out after he died just how many people's lives he had affected. How important he was to the friends that he made.
"He brought out the best in people, that was just how he was."
Aaron died on December 4, 2016. His parents, with sister Emily, 22, are today launching a Christmas appeal with Brain Tumour Research to raise awareness and funds.
Filming a video appeal for the charity at the family home, they spoke of their happiest of festive memories.
"I would always put up the Christmas tree, but Aaron and his sister Emily did the decorating," Mrs Wood told The Yorkshire Post.
"Christmas was always about the kids. Always making sure it was the best, for them. This week it will be three years since we lost Aaron. It's still so raw.
"For him to go just before Christmas, it is a really hard time of year.
"There are so many cancers, and so many have treatment or a cure," adds Mrs Wood, speaking about the campaign.
"Aaron was an amazing young man. He had his whole life ahead of him.
"To think that he's no longer here, to have a family of his own, to travel the world, to be able to grow old, and it could be because there is a lack of funding, a lack of awareness.
"It's hard to think that another family might have to go through what we went through because Aaron was taken by this cruel disease."
Brain tumours kill more children and adults under the age of 40 than any other cancer yet, historically, just one per cent of the national spend on cancer research has been allocated to this devastating disease, Brain Tumour Research claims.
Matthew Price, community fundraising manager in the North, said: “Our hearts go out to Crystal, Richard and Emily and Aaron’s friends and relatives.
"We are extremely grateful to them for sharing their tragic story in order to raise awareness of this devastating disease and to help raise vital funds for research this Christmas.”
Brain Tumour Research funds research and campaigns for greater investment in research to speed up new treatments for patients and, ultimately, to find a cure.
The charity is calling for an annual spend of £35m too improve survival rates and patient outcomes in line with other cancers such as breast cancer and leukaemia and is also campaigning for greater repurposing of drugs.
To support the appeal visit https://www.braintumourresearch.org/crystal-richard