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I think crossdressing for Halloween is one of the reasons Halloween is so loved by adults: the rules don't matter!
Remind your mom that Halloween is for having fun, and that you're not getting any younger so why wait to go all out on your outfits and enjoy?
Besides, we all dress up as something we aren't, different species, different genders, different races. Mom has the right to choose not to dress up, or to dress up in outfits that match pretty much who she is in real life. But you also have the right to choose a different path.
I think Cleopatra would approve too ;-) She sure never let being a female stop her from doing very male things in her life!
You'll have to look at how your mom typically responds to things to know what's best on how to tell her. Will she want advanced notice, or will that just give her time to worry more? Do you want her to be part of the shopping and getting stuff together, or would you rather show up and surprise her? For most parents, giving them some notice ahead of time (a day or two?) is probably a good balance between "giving them notice but not giving them so much time to panic or make me miserable" versus "giving them a near heart attack by springing it one them."