Here's what I have, data from about 30 years back when I was new to the sport and a bit obsessed with collecting data and understanding the sport.
1. Two pages from I guess the Italian skydiving magazine in 1992 listing the early tandem fatalities in order. (Apparently there was also one on a European Galaxy rig sometime in that era, that isn't in those stats.) First tandem fatality is shown as the one with one snap not done up, TI spent entire jump trying to hook it up, without deploying anything (even the drogue).
2. An html file from UPT listing tandem fatalities and causes in order (but with no dates). From 2008, back when they listed that on their web site.
3. Scribbled pencil notes of mine from the early 1990s, where I listed a bit of the evolution of tandems (eg the date when Strong & Booth got their FAA exemptions to allow tandems), plus all the early fatalities and dates. I think most of the data on dates comes from an article in Parachutist,July 1989. (Which I can't find at the moment.)
That list puts that first tandem fatality -- with the snap not done up -- as Oct 19, 1986.
Back in those days people were wondering what the heck was going on with Vector tandems, as in the first dozen fatal accidents, there were 10 on Vectors, and 1 on a Galaxy, before there was 1 was a Strong Dual Hawk. There sure were a lot of different causes of accidents -- Stupid stuff we're not allowed to do with tandems now, main bags that much more easily came out of the container early, plus a lot more casual training and attitudes towards tandems. Which brought in the idea that 'it isn't just another skydive'. It also took a while to come up with the first tandem CYPRES, and eventually the idea of making one mandatory for tandems. (Wish I had the dates for those.)
RWS tandem fatality list as of Feb08.htm