Day of Younger Sister, (Younger Sister Day) is a calendar event in Omake. It occurs every 9/6, Sixth day of the nineth month.
It can also be started by eating a Sister's Love Filled Lunch(?) that has the It Brings an End enchantment. Or by a younger sister invoking Ragnarok , they will invoke Yougner Sister Day instead.
During younger sister day, almost all NPC spawns are younger sisters. Areas affected include: Dungeons, Shelter NPCs, Party Time!, Hunting, Panic, and Wilderness.
It also spawns giant younger sisters.
In Omake Overhaul, if younger sister day occurs in the Void, the void will only spawn giant younger sisters, scaled appropriately to the void.