This can be completed at any point in the game. You will need to head Northeast from your village and this area will be located in the snow covered portion of the map at the end of a river. Head into the cave and check out the Easter egg. I recommend taking a burning club with you to provide heat, so you do not need to worry about the cold.
Here is a video guide with English commentary for this achievement!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdUJx0DT7DaKz...