I'm quite pissed right now. I played P4G up to February 14th and noticed that I couldn't access the bonus dungeon even though I fulfilled the requirement of getting a certain character's social link to level 10. Turns out I have to speak to another character after beating the regular final boss to eventually trigger that dungeon in February. I've googled several times how to unlock the final dungeon and every time those sites claimed that you only need to get that character to level 10.
My last save was from before beating the regular final boss so I have to do that again and then endure almost two more months until I get to that point again.