I know that 'tu' means 'you' and that 'su' means 'its/her/his/their'.
I also know that ¿Cuando es tu cumpleaños? means 'When is your birthday?"The translation says that ¿Cuando es el cumpleaños de su madre? is the correct way to ask what your mother's birthday is. Why is it 'su madre' and not 'tu madre'?