The X days after subscription starts notification will be sent to users on the "Xth" day after their subscription started. Similarly, the X days after access starts notification will be sent to users on the "Xth" day after their non-subscription-related access started. This value can be customized with any positive integral number.
More information about it is available here.
We do not recommend setting the "X" days variable to the value 0.
This is due to our internal backend logic that is implemented for this type of notification. It's based on an automated job run during regular intervals where the Piano database is queried for new subscriptions and accesses matching the defined "X" days filter. If a user's subscription or access record matches this filter, the email is sent (e.g. if the "X" value is set to 3, a user who has purchased a subscription or fixed time access three days ago will receive this notification). If the days' value is set to zero, this notification may not be triggered for certain users, in case our internal job runs at an earlier/later time of the day than their subscription or fixed time access purchase was made.
Therefore, the "X" days value should be always set to 1 or higher.