I like to break up an event and reveal parts of it as the book goes along. It keeps the reader interested because you are giving them a different piece of the puzzle each time, without repeating yourself and making them bored. Even if it's not a mystery, the reader will feel like they are getting closer to finding out the truth by themselves before they reach the end.
There are of course the usual ways: the character can be driving somewhere and his mind goes back to that day.
Or perhaps you can mention it in dialog between two characters. If it's a small secret that you want to remind the reader about, you can use another character who is either about to find out the secret, or who already knows the secret and now confronts or black mails the character with the secret.
Regarding this example: Remember the X 10 years ago? That's what caused this Y to happen.
I think you need to go through your book and put more details about the "X" event - reveal a few slight details throughout the book leading up to "Y". I have this exact scenario in a book I am working on. A fire that destroys a horse stable in the prologue comes back to haunt three of my characters later in the book. But in the meantime, the characters never completely forget that fire because it caused each one of them to make promises to themselves. And while they are running around the story trying to achieve their goals, those promises are constantly being tested, and they are occasionally remembering the consequences of that fire. What the reader doesn't know, is that one of them started that fire by accident. In the beginning two of the other characters also don't know that one of them started it. Slowly the secret starts to come out and they blame each other, until the final truth is revealed. - I've deviated a bit, but my point is - maybe you need to re-look at the way you have structured some of your book - are there small clues or hints or reminders you can throw in through out the book to keep this detail in the readers mind, rather than suddenly spring it up again later.
Some people don't remember small details - I'm one of those people because I sometimes put a book down for months before I pick it up again.
But then some people do remember the tiniest details. I think you also need to think about your potential readers and find a balance.