I don't know how many people celebrate April Fool's Day anymore, but I know that I lost an hour of my morning to a well-intentioned online joke. It wasn't personal, but being a paranoid schizophrenic, I really wasn't sure about that at first. Speaking for myself, I don't do well with tricks and surprises. I have enough trouble with reality as it is.
If you know someone who is mentally ill, they may seem like an easy target for even a well-meaning joke, but it may not be the best idea. It might be tempting, if you're into that kind of humor, but please think twice about how a joke may effect someone. You could easily do more harm than good.