I reckon you should keep Sundays special. That's YOUR time, and your families.
I wouldn't want a window cleaner cleaning my windows on a Sunday.
There is NOTHING special about Sundays any more. The day of rest cobblers means nothing in todays society, unless you are a devote Christian.
If your willing to work Sundays, go for it, why not? YOUR time is when you want it to be, take a Tuesday off, or a Friday, it doesn't matter you are your own boss.
If people dont mind having their windows done on a Sunday or a Saturday in some points then why not? all you have to do is ask them. You will get 3 answers of which are going to be No, Yes, or i dont mind do what you want. You know then how to arrange your round how you like.
Those working in Cities will probably find it better to work sundays rather than getting stuck in rush hour traffic thats is the madness of mondays, or not being able to park, or paying Congention Charges in London etc - ive done it myself, work a sunday and dont work monday - its a winner in my book - and you can more done on your day off cos the banks are open, and so is your accountant etc.
Sundays are JUST another day in many peoples eyes nowadays, so why not work it. As already pointed out MOST of the high street shops and ALL the large chains are open on Sundays and they have staff working for them on a Sunday, so what is the problem with a window cleaner or another type of tradesman working a Sunday?
Work it, dont work it, its your choice.