For the Church - in typical "opposite world" fashion - your birthday is your death day. That is, your birthday into Heaven.
As a child I thought it a shame that Mary lacked a birthday celebration like the other saints' received. But that's not true, it's today. Today is the day the celebration of her entrance into Heaven.
So instead of getting wrapped up in whether she died and then was taken up to Heaven or was taken up to Heaven before death, perhaps it's nice to just think of this day as THE Marian feast day.
This day always feels a bit more festive since it's the only big church holy day that occurs during the summer, at least for those of us in the Northern Hempishere.