He also posted this about Wayne, which indicates more of a connection:[You must be registered and logged in to see this image.]
If you think his goal was to shock society as much as possible, the date's proximity to Christmas might have something to do with it. Also, anorexia causes coldness, which in winter months can become all but intolerable, which might have been a contributing factor to pushing him to action. But your idea about symbolically starting a new string of shootings is an interesting one and not one I've heard before. Whether or not he intended it, he did permanently change the mass shooting narrative.
I doubt these have anything to do with it, but here's some other attacks on that date:
1903 Meißen familicide (Ernst Julius Bienert)
1923 Drew shooting (Joseph Pullen)
1990 Honghua attack (Huang Yachun)
1993 Aurora Chuck E. Cheese shooting (Nathan Dunlap)
2005 Nagarkot massacre (Basudev Thapa)
2007 Sasebo Renaissance sports club shooting (Masayoshi Magome)
2010 Panama City school board shootings (Clay Duke)
Non-mass murder related events on that date:
557 - Constantinople, the capital of the Byzantine Empire, is significantly damaged by a 6.4 magnitude earthquake
835 – Sweet Dew Incident: Emperor Wenzong of the Tang dynasty conspires to kill the powerful eunuchs of the Tang court, but the plot is foiled.
1287 - During St. Lucia's Flood in Northwest Netherlands the Zuiderzee seawall collapses with loss of over 50,000 lives. Fifth largest recorded flood in history.
1774 - First incident of American Revolution - 400 New Hampshire militiamen successfully attack Fort William and Mary
1812 – The French invasion of Russia comes to an end as the remnants of the Grande Armée are expelled from Russia.
1814 – War of 1812: The Royal Navy seizes control of Lake Borgne, Louisiana.
1918 – Portuguese President Sidónio Pais is assassinated.
1940 – Plutonium (specifically Pu-238) is first isolated at Berkeley, California.
1963 – The dam containing the Baldwin Hills Reservoir bursts, killing five people and damaging hundreds of homes in Los Angeles, California.
1999 – Torrential rains cause flash floods in Vargas, Venezuela, resulting in tens of thousands of deaths, the destruction of thousands of homes, and the complete collapse of the state's infrastructure.
2003 – Pakistani President Pervez Musharraf narrowly escapes an assassination attempt.
2008 – Bush shoeing incident (I now feel somewhat obligated to start a rumor that the shooting was a misguided attempt at celebrating the shoeing's anniversary.)
More are available here and here.