YMMV / Advent of Code

• Difficulty Spike: There seems to usually be one or two per season that both community consensus and statistics indicate to be significantly harder than the surrounding days. ^note 2015-19 (part 2), 2016-11, 2018-15 (mostly due to how many implementation details and tiebreakers there are to screw up), 2019-22 (part 2) Puzzles released on weekends also tend to be harder.
• Self-Imposed Challenge: Many people take the opportunity to learn a new programming language using the puzzles. Another popular challenge is solving every day in a different language - you will quickly run out of the most common languages and have to reach for something you haven't used (much) before. Certain individual puzzles lend themselves to custom challenges, such as visualization of the solution. 2016-5 can be visualized as individual characters of a password locking into place as they are found, just like in Hollywood Hacking.