Fifteen years ago, when I set out to write a book about the Holocaust experiences of my father’s family, I knew I didn’t have forever.

At that time, my father and his two sisters—all of whom endured Nazi concentration camps—were still alive. It was remarkable enough that three siblings from one family survived the camps; it was even more extraordinary that they would still be living, all with excellent memories and a willingness to talk, more than half a century later. Recognizing this rare opportunity, I decided to tell their story.

At first—naively—I thought my 30 hours or so of videotaped interviews would provide all the material that I needed. I could not have been more wrong. Almost every paragraph that I wrote required follow-up questions to get the details that I needed and to ensure I was telling the story accurately. My father and my aunts never tired of my questions and they were proud that I was telling their story—but as the years passed my aunts would tease me about how long the writing process was taking. “We won’t be around forever,” they would say.

They were right, of course. Two years ago, when my book was three-quarters completed, my 91-year-old aunt Lilly was diagnosed with cancer, and told that she had only weeks to live. I knew I couldn’t finish the book that quickly, but I promised her that I would have her liberated before she died.

In the following days I wrote as fast as I could, in prose of poor quality and questionable grammar, to get Lilly, her sister Oli, and their mother out of their camp, on to their long, cold march away from the front, through a bloody battle between the Americans and Germans, and finally to their liberation. Cleaning up the writing could wait; I just wanted to reach the moment when Lilly was free. As soon as I typed the final words, I called my cousin—Lilly’s daughter—and asked her to hold the phone to Lilly’s ear. I could hear Lilly’s raspy breathing. I told her that, in my narrative, she was free—that I hadn’t gotten her home yet, but that she had been liberated—and then I read her the paragraphs I had just completed. I think I heard her breathing quicken. Crying, I told her that I loved her, that she was a hero, and that future generations will read her story and admire her courage.

She died a few hours later.

When I went back to complete her story, I went through my video transcripts and my notes from conversations with her, but there were details about what she saw and felt that I will now never know. In losing her, I lost parts of her story, and we all lost one more tiny piece of the historical record of the Holocaust.

My father and my aunt Oli are still alive. She is 92 years old now; my father is 86. He was 17 when he was liberated in 1945, which was very young to survive the camps. Generally, prisoners not old enough to work were killed when they arrived at Auschwitz and other extermination camps, so my dad was among the youngest prisoners in his slave labor camp. This means that he is at the tail end of the disappearing survivor generation.

This year, which marks the 70th anniversary of my father’s imprisonment, is also the centenary of the start of World War One. There is no one alive to bear witness to that terrible conflict any more: all of the veterans of the first world war have died. Before we know it, the same will be true of the Holocaust. We may have a wealth of books and videos, but the personal connection with survivors will be gone. There will be no more chances to ask our questions, to express our admiration, or—as the students at the high schools where my father speaks often do—to offer our hugs.

My father can still give a compelling and detailed account of his imprisonment in Auschwitz and other Nazi labor camps. We are lucky that we can still listen to him and other survivors of the Holocaust. I know intimately the difference between the understanding that comes from watching a videotaped interview instead of being able to talk with, encourage and love a survivor.

So I will make the most of this Holocaust Remembrance Day. I will hold my father’s hand as we answer questions and as he tells his stories. Then we will call his sister Oli and tell her that we love her. I treasure every moment I have left with them. The rest of the world should, too.