Billing date to avoid recurring billing at the higher rate

Hi XBox community,

XBox are doing a deal on Game Pass Ultimate for £1 for a month, so I got it for my son under the agreement that we're not renewing it after the reduced rate.

My question is when do I cancel to avoid a subsequent payment of £11.99 for the second month?

The dates are as follows:

My last XBox Live Gold payment occurred on the 9th April, and the next payment is due on the 8th August.

Game Pass Ultimate - purchased on 22nd April.

So does my one month Ultimate deal end on the 21st May (one month after I bought it) or 8th May (the end of the first month of my billing cycle)?

Thanks!

:)