This is a challenge cache, so you need to fulfill the challenge requirements before logging. The cache is located at the posted coordinates.
The condition that you need to fulfill in order to log this challenge is that you must have logged at least one unknown cache (mystery) on each date of the year - 366 different dates. The logs don't need to be on the same year or on dates immediately after each other, though. This kind of challenge is a long-term project. I'd even recommend that you don't try to fill all your holes in a single year if you have lots of them. Put the holes you need to fill in your calendar and fill them by logging mysts when it suits you, and sooner than you think you'll be qualified for this challenge!
There is a checker available to check if you are qualified to log this challenge: