But I need it

Is there a class that you need to have; maybe because you are a senior or because your work schedule will only allow you to take that 1 late class? Getting an override from your professor may not be the best route. Talking to your advisor may get you further than speaking with your professor, first because 20 other students are probably asking for that same override and second because your advisor can keep tabs on students dropping the class and in some cases they are also able to open a spot for you in the class. The best guarantee that you will get in a class is choosing your classes early and keeping an eye on your enrollment date (because they are not the same for everyone).