The hat is awarded for participating on a new site. For those who like lists, here are all the requirements:

• Visit a site where you have never posted a question or answer before (even if it's been deleted).
• Post a new question or answer during Winter Bash.
• Get its score to at least 3.
• Wait until the post is 3 days old.
• The post cannot be deleted or community wiki, and if it's a question, it cannot be closed.

As others have found out, the hat is awarded for participating on a new site, which is actually a trigger we've used in the past (Winter Bash 2015's Explorer hat), which was ironically not even a secret hat. For those who like lists, the requirements for the hat were:

• Visit a site where you have never posted a question or answer before (even if it's been deleted).
• Post a new question or answer during Winter Bash.
• Get its score to at least 3.
• Wait until the post is 3 days old.
• The post cannot be deleted or community wiki, and if it's a question, it cannot be closed.