Your niche determines it if you have a site about holiday it won't but if you have a site about skiing it will these are too extreme examples.
well summer is traditionally holidays season the world over, so unless you have very specific niche reas which caters for this, you will see a slump in traffic.
I used to think Summer would bring in more clicks with everyone off school and work etc, but loads of people tend to play out and go on holiday. We may actually see more people on there Computers this Summer and more traffic, because of all the bad weather in some parts of the world at the moment.
I think in general, website traffic does slow down a bit...probably due to so many people going on Holidays and enjoying the sun instead of sitting in front of their PC. Gaming and travel sites tend to go up during the summertime...at least for some of our website clients.
I start out my site at the start of summer so haven't notice this fact. However, I am sure that most video site still have a very constant traffic.
my site not being influenced by the holidays (because it's already got low traiffic ) . for those travel site it might be opposite coz they can get more customers.
I don't think there is huge summer effect because most people wills till be at work through the sumer. Sure, it will go down by a bit, but not by much as some would make out.