Maybe the wizards found it before Columbus did.
Regardless, the first occurrence of pumpkin juice is during Harry and Ron's epic car ride to Hogwarts:
"He [Harry] had stopped noticing the fantastic cloud shapes now and was thinking longingly of the train miles below, where you could buy ice-cold pumpkin juice from a trolley pushed by a plump witch" (Chamber of Secrets 72).
Dinah Bucholz includes a recipe for Pumpkin Juice in her cookbook, as well as in the digital collection of summer recipes published last June. So that is the recipe I'll use today.
You Will Need:
1 cup of cooked and mashed pumpkin (or canned pumpkin, which is what I used)
2 cups apple juice
1 cup white grape juice
1 cup pineapple juice
2. Chill in the fridge until very cold.
3. Serve over ice and enjoy.
I have to say, this quails in comparison to the pumpkin juice sold at Universal Orlando (and theirs is basically just apple cider mixed with pumpkin and spices). You can't even really taste the pumpkin in this recipe. Honestly, if I had to describe the taste... it's closer to apricots than anything else. Luckily you can purchase it from the website above!
I recommend skipping the homemade pumpkin juice.
Go have a butterbeer instead.