Hello @zoosman2,
You can buy it whenever you want, install the eSIM on your phone and, when you're ready to use it, simply activate it. The 30-day period of use will start counting from the first event.
Here is the link with all the details of eSIM Travellers.
If you have any questions, we're happy to help.
Have a good trip!
Thank you,
Maria_Vodafone