The validity of the sim card starts immediately after receiving the confirmation of activation.
Example: You have a 30 days Plan. If you schedule the activation for June 1st it will be valid until June 30th (23h59 UTC-5 – New-York time).
If you select an activation date for today or tomorrow your activation will be processed within 24 hours maximum (eSIM is activated within few hours).
The first day of activation count as day 1.