Hi anahi78,
Yes, the 5-minute warning is a real thing
Proctors will always announce it 5 minutes before your time is up, but you should nevertheless keep track of time on your own. You can do that using an analog watch only (no digital devices are allowed on the test). Also, you don't want the warning to come as a surprise, and you certainly don't want to start rushing when you hear it. Just proceed at your normal pace, but keep in mind that you should be wrapping up soon.
Let us know if you have any other questions, and good luck on Saturday!