No founder, announcement, or establishment record for this date has been identified. The observance is kept by readers of the Harry Potter series rather than by the rights holder.
The weeks that were never in March
The first House Pride Week began on a Monday in November 2012, running Hufflepuff, then Ravenclaw, then Slytherin, with Gryffindor last. A second came the following April, timed to the third Pottermore House Cup, and gave Gryffindor the Tuesday.
The autumn 2014 edition is the clearest of the set. Pottermore's own announcement was headlined with its start date, Tuesday, September 16, and that was Gryffindor's day.
The following winter Pottermore took house pride to Orlando instead, running a Proudest House Contest across a weekend at A Celebration of Harry Potter. The prize was a million house points, awarded to whichever house posted the most tagged pride.


