DW and I just planned a last minute trip in May too. We'll be there from the 11-18. Last minute trips so totally rock dude! It's like all of the planning and anticipation and excitement packed into a smaller package.
Here's how it works from an engineering perspective:
If you consider that planning, anticipation and excitement (PAE) are compressible fluids and you put them into a container that is sized in "days". The smaller the container (the less amount of days) the higher the pressure. This assumes that the PAE is the same for a trip planned well in advance as well as a last minute trip. Now assume that your body is the container and the pressure exerted by the PAE is equivalent to happiness. Thus a last minute trip will produce a greater amount of happiness.