April 13

On March 13th, like Mar10 and Mar13 day, it spells out C4ll13 in reference to the Splatoon character named “Callie”.

On this day, you must play Splatoon 2 if you own it, no questions asked.

Glorified Cultist of Splatoon: Ayo, it’s April 13th!
Sane Individual: What the fuck are you talking about
Glorified Cultist of Splatoon: We are playing Splatoon 2

March 13

On March 13th, like Mar10 day, it spells out Mar13 in reference to the Splatoon character named “Marie”.

On this day, you must play one Splatoon game featuring the character.

Person 1: Yo, man it’s March 13!
Person 2: Damn, already?
Person 1: Yeah, let’s play Splatoon 3.