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.