When you think of sunglasses, you think of warm, nice weather, spring and summer months. Typically the month ranges from April to September, but actually the summer months can vary throughout the year based on where you are in the world.
A good thing to do before you begin your season is to research the sales throughout the previous years of where you are selling your sunglasses. It is also a good idea to check other places too, then you can close up shop for the season and take it else where and sell sunglasses all year long.
This site: http://www.google.ca/trends?q=sunglasses shows you a graph indicating the peak times of when sunglasses are in trend. For example, here is a graph indicating the peak seasons for sunglasses throughout USA:
You can see the months are normal as in Canada, because the seasons are the same, but if you were to look at Australia; the peak months are the complete opposite.
When you go to this link, you can also check by state/region/territory. By checking this graph; you are preparing yourself for the upcoming season, setting yourself up for less surprises (loss of sales) and it also might inspire you to sell somewhere else for the slow season in your area.