Typically it'd be September or October, but can be any time they feel like. Official notice for such events is usually about a week to my recollection.
In the context of the mini, look at the 2010 redesign that had no keynote/announcement. The 2011 spec update was also quiet if I remember right, and the 2012 spec update was during the media event announcing the iPad mini (and other devices).
(Just curious and not to hijack, but for a desktop system what's the significant benefit to Haswell vs Ivy Bridge other than graphics? Specifically, what would be the effect a typical user (not enthusiast or creative professional) would experience? I'm curious what reason Apple might have to release a Haswell mac mini with Broadwell coming up relatively soon?)