If you choose a Saga Premium subscription, payment will be charged to your App Store Account at confirmation of purchase or, if eligible for a trial, at the end of the trial period.
Information about the free trial (if eligible) will be shown on the subscription screen in-app before purchase.
The subscription will automatically renew unless turned off in your App Store Account Settings at least 24 hours before the current period ends.
If you subscribe before your trial ends, the remaining free trial period will be forfeited when your purchase is confirmed.
From App Store Account Settings you can manage your subscription and turn off auto-renew.