You must defeat the December boss before the end of 12/22. On 12/23, you will get a phone call from one of your Social Link party or family members asking if you want to hang out on 12/24. Choose and spend the day with them, unlocking the following equitable accessory for the rest of the game (these do not carry over to New Game +).
Gift
How to unlock
Fancy Mini-Car (Mudo Boost)
Spend 12/24 with Yumi.
Fluffy Muffler (Resist Ice)
Spend 12/24 with Chie.
Hand-Knit Mitten (Hama Boost)
Spend 12/24 with Ayane.
Handmade Watch (Ailment Boost)
Spend 12/24 with Naoto.
Leather Keychain (Resist Fire)
Spend 12/24 with Yukiko.
Macho Medal (Resist Physical)
Spend 12/24 with Yosuke, Kanji, and Teddie.
Rugged Ring (Divine Grace)
Spend 12/24 with Ai.
Silver Bangle (SP +50%)
Spend 12/24 with Rise.
Completion bonuses
Successfully complete the game to unlock New Game + mode with a cleared saved game file. In this mode, Death sometimes hides inside treasure chests. You will know when this happens if you are prompted to open or ignore the chest. If you defeat Death after the chest is opened, you will get the ultimate weapons for the characters.
Successfully complete the game with the true ending. Start a game in New Game + mode. A new Fusion section will be available, allowing you to fuse Izanagi-no-Okami.
Evolved Personas
Each character is capable of evolving their personas into a stronger form. In order to do this, you must max out their respective social links. Yu's evolved persona is first unlocked by reaching the True Ending, but must be fused in New Game+ to be fully playable.
Persona
How to unlock
Amaterasu
Max out Yukiko's social link.
Izanagi-no-Okami
Reach the True Ending.
Kamui
Max out Teddie's social link.
Kanzeon
Max out Rise's social link.
Rokuten Maou
Max out Kanji's social link.
Susano-o
Max out Yosuke's social link.
Suzuka Gongen
Max out Chie's social link.
Yamato Takeru
Max out Naoto's social link.
God Judgement
Fuse a persona during the fusion forecast "Skill Change". Effects = 1/2 HP gone from enemies. Light.
Hell Judgement
Fuse a persona during the fusion forecast "Skill Change". Effects = 1/2 HP gone from enemies. Dark.
Izanagi-no-Okami
In New Game + a new fusion section becomes available allowing you to fuse Izanagi-no-Okami.
Metal Bat
After reaching 19 Social Links, visit the Velvet Room and Margaret will award you the Metal Bat.
New Game +
Beat the game once to unlock New Game + mode.
Secret Weapons
By returning to completed dungeons, you can find the characters' secret weapons. Note: No secret weapon available for Chie. Note+: Main Character's secret weapon can be obtained early without fighting the extra boss by re-entering Void Ques before the boss appears.
Kanji's Secret Weapon - Defeat the extra boss in Steamy Bath House.
Kuma's Secret Weapon - Defeat the extra boss in Opera House.
Main Character's Secret Weapon - Defeat the extra boss in Void Quest.
Naoto's Secret Weapon - Defeat the extra boss in Secret Lab.
Yousuke's Secret Weapon - Found in the boss's den in Mysterious Shopping Street.
Yukiko's Secret Weapon - Defeat the extra boss in Yukiko's Castle.
The Reaper
Go to Magatsu Inaba in 2nd playthrough and open the regular treasure chests there (not the golden ones) , easier to find in Magatsu Inaba.
Ultimate Weapons
In a New Game +, you will sometimes be given the option whether you want to open treasure chests or not. This choice means Death lurks within. Choose to open the chest and defeat Death and he'll drop an Ultimate Weapon for each character.
Christmas Eve Gifts...