I first had glandular fever in Sept 2003, although I didn't get actual ME straight away. I was never quite the same after the glandular fever, but didn't start declining and getting bad PEM until 2006. I'm not really sure what set it off then, although it was near the start of the year so perhaps the xmas period stressed and exhausted me too much.
One thing to note: if you're looking at anything seasonal (like if we tend to get sick in winter or something along those lines) then you need to look at season over month. I'm in Australia, so I got glandular fever in spring, while I first severely declined and started looking for a diagnosis in summer. Not sure if that's what you're looking at or not, but could make a big difference to your analysis as there are quite a few Aus/NZ peeps around PR