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Pfft, the games just want it to be your birthday so they can give you musical props and other lovely things!
But really, this just means your "birthday" set on your DS is set to the wrong day (and the nurses will keep asking you if it's your birthday all day...) Perhaps you brought it pre-owned and never fixed that, or you changed the date accidentally one day. Either way, it's easily fixable if you go through the settings in your DS. But for now, enjoy the props you'll get from the Pokemon Musical Hall in Nimbasa. :)
Also, as a note, best to mention something like this in quick questions because it can't really lead to much discussion here.