If you want to make hash browns ahead of time you could do so and then reheat the hash browns again in the oven.
Just cook the hash browns like you normally would and then place the cooked hash browns in a freezer safe container and then freeze them.
Then when you're ready to serve them allow them to thaw for at least 30 minutes and then place the frozen cooked hash browns in the oven at 350 F for 10 to 20 minutes or until they are reheated back up.
Just make sure to keep an eye on the hash browns so they don't burn in the oven.
I sometimes will make and cook hash browns and then freeze them so that all I have to do is heat the cooked hash browns back up and eat them.
But I usually prefer to shred the potatoes to make hash browns and then cook them right away to make the hash browns fresh.
Fresh hash browns taste much better than already prepared and frozen cooked hash browns.
So you can make hash browns ahead of time but most people would prefer you make them fresh as the will taste much better.