YOU SAID:
My favorite day was when I slept with my sister.
INTO JAPANESE
私の好きな日は私が妹と寝たときでした。
BACK INTO ENGLISH
My favorite day was when I went to bed with my sister.
INTO JAPANESE
私の好きな日は私が妹と寝るときでした。
BACK INTO ENGLISH
My favorite day was when I went to bed with my sister.
That didn't even make that much sense in English.