Why doesn't path through variable work?

Question

linux

Works

#!/bin/bash

mkdir /home/vasya/Backup/data

Does not work

#!/bin/bash

$dir=/home/$USER/Backup/data
mkdir $dir

Why and how to fix?

Answer 1

$dir=/home/$USER/Backup/data
change to
Or even easier

#!/bin/bash
mkdir /home/$USER/Backup/data