Having initially been implemented as a fan celebration, Nintendo officially began observing “Mario Day” as recently as 2016, according to Evening Standard. The official day takes place on March 10 every year. Why March 10? Simply because the date sort of lines up with the Koopa-stomping plumber’s name!
March 10 can be written as Mar. 10, or otherwise written as “MAR10”. This simple little portmanteau has since become a hallmark date for Nintendo fans and gaming as a whole.
Source: https://www.distractify.com/p/why-is-it-mario-day