The ending is the place where all must be revealed.
Of course, any secrets the main character is keeping might be revealed earlier. The lies that were told to the main character could come out before the climax. But it's more likely that if there's a secret or a lie that is driving the plot, that won't come out until the last possible moment.
The worst possible moment.
If the story is based on a secret, then a major player in the story must not know the secret. The other characters must keep it from him/her. Actively. But we'll all be disappointed if that character never learns the secret. It must come out sometime.
Same with a big lie. It must be kept from someone. That someone might be the reader. But we'll all be disappointed if we never learn the truth.
What secrets and/or lies are your characters keeping?