Well, it depends on your definition of "jam packed". And the reason why it will be busy is because anytime people have time off, it means more vacations to Disney. Why? Because that is the way the Mouse wants it!
I was there last Prez Day and it was busy, but not anything like Christmas time. If the week after Christmas is a 10, Prez weekend is like a 7-8. I would expect crowds to start building on Thursday and be busy throughout the weekend, including Monday.
Still, a day at Disneyland is better than a day at work!
We haven't been on the 4th of July or around the Christmas and New Year's holidays, but of all the times we have been to DLR, the busiest was Presidents' Day weekend. In addition to the locals visiting, some schools have mid-winter break.
It was busy last year but lots of hotels were sold out last year. When I search Expedia this year this is not the case and even Disney hotels are available. Hopefully this will be reflected in lower attendance than last year. Still it won't be crazy busy.