I'm going to go out on a limb a little bit.
She's my mother. I'm her only child, I shouldn't have to find out about her surgery from my fiancÚ.
Then why didn't you ask her? Or why didn't you write down the date when she told you the first time, or if she didn't give it, why didn't you ask for it? When she didn't say anything in a week, why didn't you call back and ask for the specifics?
Do you see how this is your responsibility as well as hers? She told you the general fact. If you want the specifics, you should ask for them.
If she "doesn't want to bother me", well:
a) I'm an adult. She can't keep shielding me from life (only to turn around and complain that I'm not "an indipendent woman" and how she has to care for everybody in the family but nobody cares for her)
Do you see how your insistence that she should have made sure you knew the exact date simply make it look as though you are *not* an independent woman? And how are you not actually "caring for" her?
If you were going to truly "care for" her, you would be more proactive about getting the information or deciding/offering what you were able/willing/available to do for her, so that she can decide whether to accept your offer.
If you are going to maintain that she should tell you the date without you having to ask, and that she should tell you what she needs (making a request instead of accepting an offer--sort of like asking someone to give you a gift), and you get to then ignore the whole thing until SHE calls YOU, that you aren't really providing much of a level of care?
Asking the questions, getting the details, writing them down, following up on them--that's being an independent woman. That's taking care of her under your OWN steam. It's not that comforting for her to be taken care of under HER steam (with her calling to tell you when her surgery is, etc.).