Assuming you're pro-Columbus day, these are points that can be made:
Being historically accurate, Columbus day should be celebrated because he attempted and was successful at colonizing and helping America grow.
A common argument trying to counter being pro-Columbus day is the mention of killing Native Americans, so you can mention that diseases had already existed and were the primary cause of death in the New Land rather than Natives being killed (keep in mind though that disease and murder were both factors).
Another Pro argument that could be made is the fact that yes, vikings DID find America first, however Columbus helped Americas economy, etc.
Assuming you're anti-Columbus day, these are more points:
What Christopher Columbus did to Native Americans is unjust, essentially he took land from the Natives and they were forced to convert and change their lifestyle in order to work with Columbus or were killed; brutality is unacceptable.
The day itself should be focused on the Native American victims of Columbus.
The vikings technically found the New Land.
I hope this helped a little! :)
