If you mean when in the manga miss Toboso decided to make them canon, that's when Ciel is officially recognized by the Queen and gives his speech to Sebastian about revenge uwu
If you mean when and how I started shipping them, I actually don't lol I enjoy art of them, it is usually really beautiful, and I do think they're basically canon at this point (I have eyes. It's there printed on paper.) but I prefer Cielcest over SebaCiel! Hope this is not a let-down for you tho :<