It's just one of those things that every freelance journalist (and editor) should be trained to do, but very few do. In fact, I've started doing it now on behalf of my freelance colleagues at the BGTW (www.bgtw.org) and posting the results on the website.
It's very simple. You look ahead for the next 18-24 months (so right now we're looking at 2009 and 2010) and then go to your computer or library and find out what happened 25, 50 and 100 years ago (so 1984/5, 1959/60 and 1909/10).
Then you start pitching for commissions and planning features way ahead of the pack.
That's how/why I was asking United Airlines in 2001, how they were going to mark the 100th anniversary of powered flight (the Wright bros) on 17 Dec 2003
That's how I know which Oxford Street shopping icon will be celebrating in March, why the focus will be on Hollywood a year later , and why there will be tv crews down on the Dover cliffs in July
And why I wasn't surprised this morning to receive my first press release about Cuba, highlighting the 50th anniversary of Castro's revolutionary government (which, although the release doesn't mention it, came to power on 2nd Jan 1959).