Interestingly, in spite of many individual and local assertions of July 4th (give or take a few days) as Interdependence Day -- see, for example,
http://oii.org/html/independence_day.html ,
http://www.spiritualityhealth.com/newsh/items/blank/item_4458.html and
http://www.uua.org/clf/members/connections/Summer/interdependence.htm -- the major efforts to
organize a widely-recognized Interdependence Day have been inspired the 9-11 attacks. The collapsing Twin Towers highlighted the USA's interdependence with the rest of the world.
So the nonprofit "We, The World" organization called together peace and environmental groups to build mass public involvement in Interdependence Day as a new global tradition starting September 1, 2002 (see
http://www.souledout.org/festivals/globalmeditations/interdependenceday.html ). They proposed this as the first special day of "Culture of Peace Month" which includes September 11th and September 21st, the International Day of Peace designated by the UN. Their effort was endorsed by alternative luminaries like Dr. Jane Goodall, Daniel Ellsberg, Dr. Riane Eisler, Hazel Henderson, Thom Hartmann, Dr. Ervin Laszlo, and Paul Ray.
Benjamin Barber's CivWorld -- who created one of the Declarations of Interdependence, above -- have scheduled their Interdependence Day memorial for September, as well. They figure we woke up to our interdependence the day after 9-11, which suggests September 12 as a good time to celebrate Interdependence Day. They plan a major kick-off this year in Philadelphia, home of the original U.S. Declaration of Independence. See
http://www.interdependenceday2003.org/day.htm .