SIRIUS says: "You may cancel your Subscription at any time by notifying Customer Care. Your cancellation will become effective on your next subscription "cycle date," which is the next month anniversary of your initial activation date (i.e., if you activated your Subscription on January 15th and cancel on April 1st your Subscription will end on April 15th). A cancellation fee may apply."
"You will be charged a cancellation fee if you cancel a one-year or longer Subscription during the first year of service. The standard cancellation fee is currently $75.00. Promotional offers may have different cancellation fees."
XM: Cancel monthly anytime
XM says: "You may cancel Service by notifying us. This notice will become effective at the end of your billing cycle. You are responsible for payment of all outstanding balances accrued through that effective date. We (or the creditor or lessor of your vehicle, if your Subscription Fee is included in the amount financed or leased) will refund amounts you may have prepaid on a quarterly, annual or multi-year basis representing those months beyond the billing cycle month during which you cancelled your service."