I am surprised you did not choose the 15th... tax day! Apparently the 15th is the date for the national tea party... Saturday is good though I guess because I won’t miss work...
We chose the 18th because most of us are productive members of society and are working (at this point in the year for the government) on that date.
The 18th, coincidentally, is the 238th anniversary of Paul Revere’s ride.