Do I buy 2 b-day cakes for my daughter?

Get a small cake or a couple of fancy cupcakes for Saturday for the girls to share. Many bakeries in stores have them. Have the larger cake for the family on Monday

Your daughter and her best bud are both old enough to learn how to make cakes and other baked treats themselves, so why not have that as an activity this weekend? I'd be willing to bet they'll have a blast learning how to do things like bake a cake, cookies, brownies, or whatever, and then frosting and decorating them. At the end of the process, they get to enjoy the fruits of whatever they have made.

And to add.... Cake typically isn't stored in the fridge, unless it's made with either custard, pudding, or whipped cream ( the way some cakes, such as Boston Cream, generally are) but on a cake stand or in a covered pan on the counter somewhere. Why would you put it in the fridge when you don't need to? The usual run of the mill birthday cake will do just fine stored at room temperature. It's not going to spoil or make anybody sick. And you can keep it covered in a sealed container to keep it fresh.

There is no "etiquette" involved in this. Do whatever you want to do!! If you want the girls to have something special, why not order a birthday pizza, or a big cookie--for the sleepover--then have your birthday cake Monday?  Or any number of OTHER treats for something special?  The girls could make the birthday cake themselves! They could make cupcakes!  There are so many options they're not worth listing--you can do whatever you want to do for either day. 

Sounds like a good idea to ask her for

yaself as she is @ an age where surprise 

B-day party's are kinda boring my dude

Dawg-cuz !

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For the sleep-over, a store bought pack of six cup cakes from the bakery should be fine.

For the family party - have an actual cake.

You're overthinking this! Just ask her which she would prefer. 

Personally I would get them pizza and cupcakes for the sleepover and have the cake on her birthday, but I'm sure the birthday girl's opinion is far more relevant haha

I would have a birthday cookie for when her friend comes with pizza or kids food, then for the family the cake on her real birthday.  Get a couple of birthday hats or tiaras for the two of them on sleepover night and ask your daughter if she wants a movie or game for them.

Save the cake for her actual birthday. I would just have some goodies (cupcakes maybe?) for them to share for the sleepover.  

i dont think maney 13 year olds care about that etiquette