After signup for the service, you should have received a confirmation email with the details of the subscription, including length of the trial period, and the first day of payment after the trial, if you are still subscribed at that time. That description is how it should be executed.
Please verify how it is shown on the subscriptions page of your Apple ID. That should also list the next payment day for this subscription. Manage your Apple Music subscription - Apple Support
Note that the free trial only applies to new subscribers. If you subscribed before, then you can’t get a trial period a second time.
For further assistance, I would recommend contacting the iTunes Store staff. Your invoice should have an order number; include that in your correspondence.