Just in case someone is looking for a cheap last minute solution from Europe to California consider this great option based on Swiss Air Lines and their Star Alliance partners Lufthansa/ United. You can now book cheap return flights from Europe to San Diego already for €317. This is for round-trip incl. all charges and fees however at this moment only two outbound dates are available at this low price. Look for flights departing from main airports in Germany, Italy, also from Benelux, Scandinavia or Vienna. Also you can book non-stop flight from Zurich for just €352.. 
As mentioned very limited travel options are available in June 2017. To book your accommodation in San Diego in advance you may use Booking.com. You can also use Amoma or Hotels.com promo code. You can refer to below booking samples. You can also consider to book very cheap non-stop flights from Barcelona to Oakland already for €283. Note this is low-cost solution with Iberia / Level.
Booking sample
Zurich - San Diego from €352 (Non-stop flight)
Travel samples
8. - 18.6.
9. - 23.6.
To book your accommodation in California in advance you may use Booking.com. You can also use Amoma or Hotels.com promo code.
Booking sample of Cheap last-minute flights from Europe to San Diego, California from €317 here. In this case you will be departing from Frankfurt to San Diego on 9th of June. You would be returning from USA to Germany on 24th of June 2017.
