Hi Benjistoney,
I don't think there is A website that includes all the venues and open air music concerts all over Israel (I am in Israel and I can tell you that technically it is complicated).
Nevertheless, during summer months in almost each and every City or Town in Israel there are plenty of concerts (just last week there were nice free open air concerts in Tel-Aviv and several other towns), the best idea is to do what Chosen suggested previously in this thread, in the City you will be visiting check out the local news papers, billboards, ask the locals, this is the best to get the relevant info of the upcoming shows.
If you have family members/friends in other Cities/Towns that you wont be staying at, ask them to check for you this way you will even be able to plan ahead.
A lovely weekend to everyone at Kikar-Israel
