How do I tell people I don't want anything for my birthday?

Maybe I'm a complete cynic or a hypocrite, but I really don't want to celebrate my birthday this year, at all. I love birthdays and I love giving presents to people and having a good time with them to celebrate, however I don't want anything for my birthday and I haven't made any plans and don't intend to.
I'll be 15 and everyone keeps asking what would I like and my mum and my sister want to make plans for me, but I just don't want to!
How can I tell people this without upsetting them?
How do I tell people I don't want anything for my birthday?
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