Everyday and every day do not mean the same thing!
Also, you don’t need a comma after future. You would if you moved every day at noon to the beginning of the sentence, though: Every day at noon, you find a door in the bathroom at work that turns into a portal to the future. (I changed you find out to you find because, after this happens once or twice, you’re not rediscovering the fact of the portal each time you see it.)
Notice how the emphasis and meaning of the sentence is changed somewhat because of this rearrangement and minor rewording. Before, the emphasis was on the discovery of this portal; when it opens is secondary. The changed version is about there being a portal each day.
Here’s the corrected version of the original wording:
You find out that a door in the bathroom at work turns into a portal to the future ever day at noon.