Would it be possible for you to increase your on stock supply of your most popular windows, to cover the expected increase in sales during those months, or do you manufacture them as the orders come in?

You might also consider posting a notice for your customers, informing them in advance that purchases made during this time period may result in longer than normal processing times, as a result of an expected increase in sales during this time period?

You might even consider offering your customers a discount on their next purchase, should the delivery of their current order take longer than expected for delivery?

Any advance notice made available to the customers regarding possible delays during this time period, should help to reduce the potential for a dissatisfied customer.