Regardless of whenever you purchased your eSIM it gets activated automatically the first time you connect to a network in the country where the eSIM works.
Which means you don’t need to nominate an activation date or commencement date at the time of purchase.
You will need to choose the new SIM to use in the new country. You can do this before arrival as your SIM won’t activate until it also connects with a network in that country.
Selecting Your SIMs
Your phone probably has capacity to hold more than a dozen different eSIMs at any point in time.
Most phones only have 1 or 2 radios in which can connect with networks however.
Which means you have to choose which of your SIMs you wish to use at any point in time.
When you arrive in a new country, or wish to change SIMs you need to go into your phone settings and choose which of your SIMs you wish to use.
365 Days to Activate
You have 1 year in which to activate your eSIM before it expires.