Here's what happens on this day:
* The Lake Guardians gather: All three Lake Guardians – Uxie, Mesprit, and Azelf – will appear at the Lake Acuity, marking the start of the festival.
* Special dialogue with the guardians: You can interact with each of the guardians and have special dialogue with them. This dialogue is unique to this event.
* Increased encounter rates: The Lake Guardians, as well as other rare Pokémon like Gible and Manaphy, will have an increased encounter rate around the lake.
* Special items: You can obtain special items like Miracle Seeds by talking to certain NPCs in the area.
* The Lake Guardians disappear: After a certain time, the Lake Guardians will disappear, leaving you with the memories of the event.
Note: The Lake Guardians' Festival only happens once, on January 12th of the in-game calendar. You'll need to make sure to visit Lake Acuity on this day to experience it.