Semi-spontaneous travel?

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I want to travel abroad to Japan. For the first time in my life I have a job that gives me vacation time.
I have a few thousand saved up and could easily afford a trip there, I think. I haven't done all the math for the trip, but going on a rough guess. That said I have 2 weeks vacation left. One of which is the week after next.
Should I push it off and plan a trip in the fall or maybe plan for sometime next year? The goal is to get to Japan before the end of 2018. I've never been out of the country before either. I feel I'd want to spend 2 weeks abroad, rather than 1, thus I'd need to push my current break off.
Anyone have any experience doing something like this?
Should I just take the week break and plan further out for next year? Would a semi-spontaneous trip even be worth it?