The FBI left the criminal servers running to give infected users time to remove this piece of malware, however it is estimated there are still hundreds of thousands of people who just don't know they are infected.
Monday is D-day because anyone who still has this piece of malware on their computer simply won't be able to access the internet.
This means people will struggle to repair the problem because they won't be able to download removal tools or access information through the infected laptop or computer.