One night, some friends and I pulled the dusty globe off the shelf, closed our eyes, and pointed to a random place after a swift spin. A bit like spin the bottle, except our plan was to travel to whatever destination we landed on. And we were headed to Madagascar to hang out with the lemurs. The next morning, we awoke with hangovers and decided to put our plan on the back burner. In the sober hours of reality, I wasn't really all that interested in going to Madagascar. There were just too many other places that I wanted to see first. And therein lies a problem - the desire to go everywhere.
For me, Asia was the place to start – specifically, mainland SE Asia. Knowing I could travel for 9 months, or possibly 12, I got out the map and started plotting out the journey. I thought about tacking on a New Zealand and Australia leg at the beginning of the trip... and then my list continued to grow: India, China, Japan... I figured that once I was so far from home, it would be a huge waste not to see every square inch of the Eastern Hemisphere.
I finally settled on a rough plan that took me from India to China and then to SE Asia. I gave up Australia and New Zealand and Japan because of time: I didn't want to rush. Also, I wanted to travel overland to save money. Eliminating these countries saved on airfare (and more expensive daily budgets as well). On the road, I met lots of people traveling for the same time period as me, but to far more places, including South America. They got to see more of the world, but I got a more in-depth experience of the countries I visited.
Planning the trip, I kept the itinerary loose, not knowing how long I might spend in some places or in what order, exactly, I would travel the countries of SE Asia. I decided to start with India because I'd never been there, and starting the trip in a completely unknown place seemed far more interesting than starting in a place I've been before (a few countries in SE Asia). Also, starting in India allowed me to move towards Bali, a place chosen as a 'final destination' to rest travel weary bones before returning home.
Although I had a tentative plan for moving through Asia (India towards Bali), things changed as I traveled; my plan was transformed along the way by weather, festivals, and road fatigue among other things. But that's the fun of travel: the unplanned and (sometimes) impulsive decisions that change the program, however loose it may already be.
It gets more complicated when you're not only traveling to one country, but many. You may start in the dry season here and end up in the monsoon season there. So be it: there's not much you can do about the weather when you're on the road for an extended period of time (but you can escape: I went to Indonesia (earlier than planned) when mainland SE Asia was flooding).
Other ways to determine your departure date: plan around the dates of festivals and holidays you'd like to experience overseas; plan around holidays and important dates at home (I left after the Christmas holidays and after taxes were due); plan to avoid spikes in tourism during certain time periods like Christmas.