We at Nebraska (and a lot of schools in the Midwest-ie. old Big 8 area- for that matter) have summer rush. Basically starting in April people going through rush have to submit a rush application (click here to access web based
form).
On the Friday of finals week, all the applications collected so far are then distributed to the fraternities, and they may begin making phone calls from that point. Usually during the Month of may, most houses will go and visit most PNM's and take them out to dinner/bowling/golfing or what not. Then during June, and early July houses will have rush events at various locations throughout the state, most being at lakes so that you can boat, and hangout. Most houses have 5 or 6 of these events. Also the houses are required to be open for tours after New Student Enrollment days. At any time houses can offer bid cards, and it's not uncommon for a rushee to recieve bid cards from more than one house.
I think that summer rush ends around August 15 or every year, and if you haven't signed by then you have to go through formal recruitment. At Nebraska Formal rush is sparsely attended and most houses don't go through it. There was a long period of time, like 20 years in which the university didn't even have a formal rush...everything was exclusively summer rush. Three years ago they brought it back, and the first year they had 50 guys go through, last year only 30 went through, and of that 30 only 12 signed a house. I have no idea how many will go through this year.