I remember waking up on a friend's sofa the morning of 1st January 1998 with badly swollen glands. Later that week I drove back to Bristol and I can remember collapsing on my bed with the first ever experience of the horrible post exertional fatigue, qualitatively different from any previous tiredness.
Technically this was me coming down with glandular fever (EBV/mono), confirmed by blood tests, but I don't know when this blurred into ME. So my answer is both yes and no. I definitely had a virus trigger.
I've had a lot of contact with pwme and I've never noticed a correlation with type of onset and severity. This doesn't seem like something worth spending energy worrying about.
I agree it is unusual to have specific time onset without a trigger. My understanding is that some people have a vulnerability to ME but there needs to be an environmental trigger which disrupts the body's normal way of functioning and becomes dysfunctional (perhaps as a perceived adaptive state to keep the body alive) or that the ME starts to express itself (epigenetics). Usually the trigger is a virus but appears to sometimes be other things like harmful chemicals, bacteria infection, surgery. This isn't certain but seems to be what happens.
@nikefourstar are you completely convinced that you do have ME? Do you have delayed worsening symptoms after activity? I have no idea either way in your case, but a lot of people are misdiagnosed.