When to turn on International Pass?

We're going to Europe at the end of the week for two weeks. We decided to use the international pass just to make life easier. I know we are paying for convenience, that's fine. But my question is when do I buy the pass? I've read that it automatically works, so I don't have to turn it on but I also don't want to eat into that data while I'm at home. Do we wait until the day of the flight or turn it on a few days early? Last time we did international travel we had pocket wifi, which was great, but it's not that much cheaper than the pass and it's another device to keep track of and keep charged. Not worth it this time. But I can't see how it really works or when to get it explained. Thanks for the help.