Agree with @joss , Spotify typically allows you to cancel anytime on its subscriptions and trial offers, as seen in link below therefore you would need cancel no later than the day before the offer ends, unless you want to carry on with the service and are ok with paying.
Do read the T&C’s particularly section 3 – 4 – 4.1 – 4.2 & 4.3 as shown in link and importantly check the T&C’s of your offer match the ones shown here. Towards the end of the trial, its important to follow the cancellation steps described bold under section 4.3 Trial duration and cancellation.
https://www.spotify.com/uk/legal/paid-subscription-terms/
Any problems or if your unsure if your T&C’s match the ones shown in the above link, put the link up and we can read through them and/or help further. I’d also advice making a note on a calender of when your due to cancel, so you don’t forget.