Dear Mom, I'm donating a Kidney

Okay, so that might not be the best way to start the conversation, but I think its going to have to happen. A very wise friend of mine told me that parents (no matter how old the child is) do not like to be told that something is happening, they like to be involved in the process. So by telling my mother before the date is set, and letting her go through the remainder of the process with me, I am more likely to have her support and the relationship will be better long-term. Therefor I have altered my plan of not telling them until the date is set for surgery.
Thats wonderful. That really is. Now how do I do it?

Part of my anxiety about this is I don't want this to overtake the weekend. I would just like to tell her, answer her questions, then move on. I want to see my family and spend time with them, not spend the entire time talking about this. So I'm thinking I might do it on Sunday before we leave, or maybe over dinner and a bottle of wine.

I love my mother so much, and I want her love and support. But, and there is always a but when it comes to family, I don't want her to spread this around. I don't know if I even want my brother to know. The more people that know the more likely this will get out or something, and I prefer to have it be just something I'm doing, then move on with life when it's done. And the more people that know, the harder that will be.

But, I do know that I will have more support post-op and my husband will have some help taking care of me when I am convalescing.
I guess maybe the reason I'm scared to tell her is that I'm scared I will get the "negative support" that I got from my boss and her husband.
The weekend after I tell my mom and dad, we go visit the in-laws and I will be telling them as well. Not overly worried about that just yet, but I think a lot of how I feel about telling them will depend on what happens next weekend when I tell my mom.

The question is: If they hate the idea and are really against it, how will that affect me? And will they be able to support my decision even if they don't agree with it?
Any ideas?