If you set up SQL to automatically backup your database on a schedule using SQL Agent, it will force a date increment in the backup file name. So the Northwind database would have backups named:
Northwind_030519200.bck
Northwind_030518200.bck
Northwind_030517200.bck
Northwind_030516200.bck
You need to tell the schedule when to backup the database and how many backups to keep. Also, you shouldn't name your backup files with .bak, they could get flagged as temporary by other Windows utlilities that look specifically for .bak and .tmp files.