big 4 frats

When do big 4 frats usually start contacting people about fall rush? Are there summer events and events very early on? I'm a chill guy and have some friends in the big 4, however I wouldn't call them close friends, more like friendly acquaintances who I see frequently due to classes/clubs/whatnot but they definitely seem chill with me. Problem is, most of the conversations we have are about other stuff so I'm not sure they think I am interested in fraternities and rushing. Should I try to be more proactive with casually asking about rush events coming up to let them know I'm interested? Thanks.