YOU SAID:
i am throwing a big birthday party for my sister on saturday
INTO JAPANESE
私は土曜日に私の妹のために大きな誕生日パーティーを開いています
BACK INTO ENGLISH
I have a big birthday party for my sister on saturday
INTO JAPANESE
私は土曜日に私の妹のための大きい誕生日パーティーをします
BACK INTO ENGLISH
I have a big birthday party for my sister on saturday
That didn't even make that much sense in English.