I second what the unlimitedposting andghands have said : Holidays change nothing to internet activity.
Some people might take a couple weeks off, but they won't leave for two months, so only a part of them won't be online on a particular day or week.
Some other have more spare time to spend on the internet during holidays (especially students).
But a huge part of internet users change nothing to their online behavior because it is summer.

So in the end, there is no more variation in internet activity during summer than during any other period of the year.

Then your problem seem to be with your website (onsite tech error or something with your traffic sources), but that's good news because once you've found what it is and how to fix it, you'll have it running again in the summer as good as you had in the winter