When does the plan’s validity start?

It depends on the plan, and its description says plainly which rule applies:

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  From the first connection to a network at your destination (most plans): you can install the eSIM a week early, and the 7 or 30 days start counting when the phone first connects to a network in the destination country.
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  From installation: the countdown starts the moment the profile lands on the phone. Install a plan like this no earlier than the day before you leave.

A regional plan that covers Poland can connect to a network while you are still at home, because Poland counts as “at your destination”. Turn the eSIM line off after installing and turn it on once you have left; the guide shows where to do it on each phone.