I am hearing "crud monkeys! I forgot Mom's surgery! She didn't tell me the specifics, either! Is it on her, or on me, and what do I do now?"
I suggest calling her and telling her your heard the date and want to know if she needs anything from you around this surgery. It is, IMO, actually, on both of you, so stop worrying about it, and, later, remind her that is it your joy and privilege to help her when she needs it, and you'd like to be kept in the loop as you are never too busy to help her when she needs it.