actually the day of 9/11 is a sad & a day of mourning in both worlds.
in the physical world-WE are in-you should mourn the loss of over 3000 lives R”L while:
in the spiritual perspective you need to face reality & ask yourself:
do you know why 9/11 happened? i.e. every tragedy that occurs in klal yisroel has a wake-up message STRAIGHT from Hashem. do you know what the message of 9/11 is?