What is the purpose of opening this package in March?
You should have opened door 5, not box 5. You can tape box 5 back up and continue with the game with no ill effects, especially as the meaning of the contents of box 5 is not apparent to you at this time.

From what I remember, when you are instructed to open a box (or package as the game refers to it), it will specifically say "Open Package #". Numbers in circles are doors to be opened, and letters in circles are rules to be opened (same mechanism as doors).

According to this thread, the contents of door 5 is


   Three additional scars, so you're not missing anything vital