Subscriptions renew automatically, and you might be charged 24 hours before each subscription period starts. This why it's important to cancel subscriptions early enough in order to avoid unwanted charges. The Premium subscriptions are not eligible for refunds. 

The subscription continue until the end of the subscription period no matter when you cancel it. 

You need to cancel the subscription in 
Google Play > Account > Subscriptions. 
Or alternatively with a browser by going to  Google Play > login > Settings/ My account (upper right hand side) > Order History > More (...) > Cancel subscription

You can manage your subscriptions also in here

Free trial:
Please note that the free trial is valid for 30 days and will continue as paid service from thereon if you don't cancel it. The trial lasts from the exact time you start the trial and 30 days onwards, so please be careful with the time. If you don't want to continue with Premium after the trial period, we recommend cancelling at least a day before, the trial will continue until the end, no matter when you cancel it.