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Spotify Premium is always paid for in advance, meaning that if you cancel your subscription at any point, the automated payments will stop, but your subscription keeps going until the month you paid for ends.
If you use a pre-paid payment method to buy 3, 6, or 12 months of Premium, you don't have to worry about cancelling the subscription. It'll go back to Free automatically once the pre-paid months are over.
Hopefully this answers your question.
“Music acts like a magic key, to which the most tightly closed heart opens.” ― Maria Augusta von Trapp