YOU SAID:
what are you doing on you 15th birthday
INTO JAPANESE
15歳の誕生日に何をしていますか
BACK INTO ENGLISH
what are you doing for your 15th birthday
INTO JAPANESE
15歳の誕生日に何をしていますか
BACK INTO ENGLISH
what are you doing for your 15th birthday
That didn't even make that much sense in English.