Archie,
You may find the odd customer will say, can you stop and return in April? Laugh and walk off. You will also find that since you are new, you will have a higher percentage of 'bad customers' than is normal; you'll get the stuff that other window cleaners have binned.
But the vast majority of people realise that if they want your service - and they will - they will want their windows cleaned all year round. People who live in tidy houses, have tidy gardens, and keep themselves tidy, will want clean windows all year round.
Just think of all those bright crisp Winter mornings; people want clean windows even in Winter.
And on the run up to Christmas, it gets really busy, especially with people wanting inside cleans because they've got family and friends visiting and they want their property looking perfect for Christmas.
So don't worry about it. We did have a tough Winter at the start of this year, but that was a freak weather event I hope.