My first "DO" was the LJU (Ljubljiana, Slovenia) DO. I went alone. I knew none of the attendees personally. I did know one by name in advance, with whom I exchanged a few PMs regarding a Coupon Connection exchange about a year earlier.
I had planned to be in Croatia around that time and since I would have to fly into LJU to get there anyway, I thought it would be a good way to meet other FTers and see a city with a group of individuals with which I had at least something in common--FT. FTers are a most welcoming group of people, I never felt like "billy-no-mates."
That DO was almost evenly split between women and men; there were married couples, married people without their spouses, and single travellers, both male and female.
We explored the city together over 2 days and shared our evening meals at a pre-selected restaurant. There were impromptu parties in hotel rooms after dinner, which were open to all, but no one was required to attend. As a woman, I found it especially nice to be able to wander a foreign city at night with other FTers, as that is something I would never do while travelling solo.
It was also the DO where I was dubbed "Pat with a bunch of numbers" since there was another Pat attending and it made it easier to distinguish us.
As for the term "DO," it is a slang expression for an event or something "to do."