Riddle Me This #7

This is my first Riddle written for the new blog and my first in months [I repeat, months] which should explain it's crappiness. Here goes nothing and leave your guess in the comments : )

RIDDLE 7

I am used for leaving
Overnight
A Long Weekend
Permanently
If small, I am overhead
If heavy, I am below
I am necessary only when you need me
Sit on me if I grow.
Who am I?

Hmmmm, any guesses from my three faithful followers? ;)