My cousins are Orthodox (black-hat/yeshivish/live in Passaic) and one of them is having a Bat Mitzvah in November. I have never attended an Orthodox Bat Mitzvah, so I don't really know what to expect.

They are having it on a Sunday afternoon at their house (not Rosh Chodesh). Is this typical? Since girls don't read the Torah portion maybe it's ok to have a bat mitzvah on Sunday? What will they be doing at their house? Why would it not be at synagogue? Would it be the kind of thing where I should dress up like on Shabbat or are regular clothes (following the Orthodox modesty rules) ok?

I'm sorry if this is a very obvious question.