Everyone's shop is different when it comes to busy times and slow times because it depends on what you sell. Once you have been open for a year or so, you will probably get an idea of when your slow times are.
Since the summer is usually slow for me, I like to take this time to create new products, work on my SEO, shop appearance, etc.
Because I'm constantly listing new items and promoting during the summer, I actually do get some sales coming in. So it doesn't hurt to take this slow period and get productive. You never know, it might help give your shop a little boost.