Anniversary date on facebook?

I went from single to in a relationship (after a week break up) and want to put my anniversary back up, I had it before, my gf doesn't have a fb account anymore but before we broke up I had our anniversary and relationship status up. Why isn't it letting me put it back up, I have other friends who have it up and their SO isn't on fb either, and 1 who has yet to confirm the relationship yet.