[BC] Closed my account, Shaw wants me to pay for another month after closing

I switched to Teksavvy - their service starts on May 21st, I emailed Shaw on May 17th to have my account cancelled on May 20th, I got a reply saying cancellations have to be done over the phone, (even though CRTC requires operators to cancel upon request). Fine, called today, said the same thing, they didn't even try to keep me, they just told me how bad TS is and how great Shaw is, but whatever. My billing period is from 21st day of each month till 20th day of each following month. They didn't drop the charge for another month which starts on the cancellation date and still want me to pay for a service I'm not going to use. I need explanation, why do you still want me to pay for a month that starts on the service cancellation date?