If you are writing this sort of an essay, your main point will probably be that Columbus's "discovery" of the Americas was a terrible thing for the native population. Your grabber sentence should reflect that idea. You might also want to contrast the impact of Columbus's "discovery" with the kinds of things that actually should be celebrated. If you do this, you will be able to set up the main point of your essay in an interesting way.
For example, you might say
Celebrating Columbus Day is similar to celebrating the days that Adolf Hitler or Joseph Stalin took power in their countries.
By saying this, you would be (in a rather dramatic way) saying that Columbus's impact on the New World was similar to the impact that those men had on their countries.
Or you might say
Holidays should honor events like the signing of the Declaration of Independence that actually improved the world, not events like Columbus's "discovery" which led to the destruction of entire civilizations.
I hope that gives you a place to start. Good luck!