Summer in Europe is the dead season, especially July-August. All hires are stopped. All projects are paused. Everyone is on vacation. It's a complete Mañana time.
When you're working for European clients, you need to plan what you're going to do in the summer ahead. If you're paid by hours, you're at risk of receiving much less during the summer season. I've been there and I know how to avoid that.
The first and the best solution for the idle problem is a retainer. With a retainer contract you will be paid for X hours/week no matter what. If your client doesn't have enough work for you, it's their problem. You will get your compensation for X hours anyway. In case if the client has more work for you, you will simply send an additional invoice for extra hours on top of those secured X.
On Upwork, for example, you can implement a retainer contract by creating a new milestone or adding X manual hours every week.
Multiple different clients
This is about diversification. To always have some work to do, combine your European clients with American ones or add some startups to your B2B pool. On top of completely different work approaches in different countries, small startups work a lot, ignoring holidays and vacations unlike big corporations.
Make yourself busy
If you have some savings, you can just have some rest with your family during a holiday/vacation season or...
You can quickly run your own project. It can be a course, a book or a small SaaS service. The important thing is to treat it as a quick side project until you see a strong indication of it going to earn enough money.
Hope these ideas help you get rid of the idle time completely or at least prepare for it properly.