According to the archives on the official site, after the family buries Mr. B under Farley's tree, Liz starts stressing because she got an invitation to Anthony's wedding and doesn't have a date. The sequence wherein she gets hooked up with Dennis runs from Tuesday, July 8 2003 through Saturday, July 19 2003. (The dance lessons come later; this is mostly her discussion about "platonic" dating and so forth.
However, in the last pages of the book, we jump from 7/7/03 and John's quip that he "wooden know" what trees think about, to April, on 7/21/03, bleating, "What do you mean, I'll be staying with Gord an' Tracey?" The remainder of the book is taken up with John and Elly on vacation, canoeing and discussing retirement.
I HOPE Dennis isn't going to be left out of the books just because he's...you know...GAY. The 8/2/03 strip is the last one in the book, and the 8/3/03 strip has Dennis teaching Liz to ballroom dance. So it wouldn't be a glaring continuity gap if the strips that explain his presence and introduce him start off the next collection. Anyway, I don't see how he could be left out, without skipping Anthony's wedding entirely, and if that happens, we have no context for Evil Therese. But I just thought I'd mention it.