You will automatically open a Mystery Tackle Crate after doing 6 FISH NOW fishing trips or 6 SKIP's.
UPDATE: You can now pick whenever you want to open it, just click the Mystery Crate link on the main page
, and click the Open NOW button. It's a daily reward for doing various daily tasks. Similar rewards to the 1st and last MTB that you open each day.. but different daily actions effect your luck of getting a better reward, such as: Tournament Casts, collecting crates for consecutive days, making quest catches, taking crew fishing, night fishing previous day, opening MTB's, entering the raffle and visiting the forum.. along with a couple secret luck multipliers that I'll let you guys figure out once coded in. All of the daily actions have diminishing returns on luck.