I'm being pretty hush hush about my surgery. I recently told my boss I would need to take time off. I didn't tell him why and legally he can't ask. I adore my boss and have worked for him for years and consider him to be one of my best friends, but some things just need to be private.
I just told my Mom about it and was really nervous to do it, but she took it ok (I asked her to tell my Dad). I'm waiting until after the surgery to tell other family and probably won't tell most of them unless they ask me point blank.
As far as friends go, only one knows.
By the way, if my boss does ask me what the surgery is for since we are friends, I plan on telling him I would like to keep it private. I know he will understand, but I also think he may ask my husband who has been sworn to secrecy.