If this is your maiden trip to Ibiza then i personally think it's probabaly a good thing you're not travelling right in the middle of silly season (mid-July to end of August as Evolution says).
Not only is that when the clubs are at their most mental and packed to the rafters but it's also when you get the big crowds of youths descend on the island, because it's during school/college/uni holidays and the atmosphere is somewhat, erm, "
different". Even for a seasoned-vet that can be a little too intense, and put you off. Not to mention give you an inaccurate perception of Ibiza.
The fact that there
isn't going to be
loads of parties on at the time when you go will actually do you a favour imho. I think it will make you go to a party you perhaps wouldn't normally consider. And sometimes going outside your comfort zone and experiencing new things is better than sticking to what you know - and that is especially true in Ibiza!
You will have a great time still, and then, after you fall in love with the island, it will give you better scope to what you want to do on your second visit to the island (which you will probably start planning the minute you arrive home!)
If last few years are anything to go by then I would expect Pacha's weekly line-up to be pretty much started by then - although this year marks a big transition for them, so that could of course change as much as the line-ups. And there will be events going-on at the other clubs, just that they might be one-off parties as opposed to weekly fixtures.