For those who haven't heard of it before and those who have forgotten, last April Tomasz Steifer wrote the following explanation for the proposed date:
Polish Heraldic Community , in the persons of the representatives of the Board, Tomasz Steifer, Vice-President of PHC and Pawel S. Towpik, Secretary of the Board, proposes to create a new tradition - the International Day of heraldry. This was to a great, over political, international opportunity for meetings, conferences and taken away heraldic organization, heralds and heraldist of different countries and continents, to the posting of flags, especially heraldic, dress in heraldic tunics and tabards, etc.
We propose that this international day of heraldry at 10 June. On that day, in the year 1128, in Rouen was knighted, by his future father in law, Henry I Beauclerc, Godfrey Plantagenet. Suspended during the ceremony on the neck of a young knight shield blue decorated six golden lions, is recognized by most of the heralds, for the first time in history, fully formed coat of arms.
Immediately after the New Year I propose to write to the heads of all the Heraldic Authorities encouraging them to do something special on 10 June. For example the College of Arms might hold an open day. But I would hope that we could do other things to ensure that 10 June becomes known as International Heraldry Day. Last time Jeremy Hammond did something on Facebook and Twitter. Could we prevail on you to do the same in 2014 Jeremy? Other ideas are very welcome, but don't feel you need to ask for permission, unless you want to use the IAAH as the vehicle for whatever you have in mind.