Dear Chip,
As Norman says: you can use your Direct Payments to employ anyone you wish (except a member of your family who lives at the same address), whenever it suits you best, but be warned: 10 hours / week is at the most basic rate. Some agencies charge more per hours than others, and rates vary quite a bit from weekdays daytime to evenings, Saturdays, Sundays and Bank Holidays. Your 10 Sunday-hours may, therefore, make a big hole into your allowance, but perhaps you are able to 'save' a few hours during other weeks, or top up the Direct Payments with your own funds to cover your special Sunday.
I hope you can sort it out and that you will have an enjoyable day.
Just a thought: if you need cover for a special occasion, you may be able to get some funding from Social Services for yourself (as the carer). I am assuming that the Direct Payments are in place for your wife/partner.
Let us know how you get on.