Definitely before you leave, as others are saying! I did it a couple weeks before, but for a specific reason. Found out the old phone I planned to use for my 8yo (who doesn't have any ipod or anything) wouldn't install it, so I had time to borrow an old one from my in-laws, clean their stuff off it, and make sure the app worked on it. My mom didn't do it ahead of time, which I learned while we were in the embarkation line for photos, and I figured I could quickly do it b/c it was so fast on ours at home. Uh, no. it took forever!! We were sitting down to lunch by the time that darned thing was done installing. It made me re-boot her phone, etc, but the actually install was super pokey.