I only usually give customers an approximate date for the next clean and say to them it could be a few days before or after.
Some of my work has been a week or so behind of late but most customers don't seem to mind and are usually glad to see you.
Very occassionally I will ring customers and let them know I am running late but I don't usually bother
Just try and fit as much work in as you can when the weather conditions are not too bad