As long as you can get in to the building, you can get in to the labs with your access card. Once the building officially closes, if you step out of the room, you will be locked out. If you happen to trip an alarm (i.e. leaving door open, opening the door when the sensor above it is RED), security will come and kick you out.
ITB has Saturday hours for sure. Sunday, I'm not fully certain of but I can't see why not.
says thanks to eullwm for this post.