Lyric discussion by Stervenson 

Cover art for sailing by night lyrics by Department of Eagles

I'm pretty sure that the lyric isn't "left for your shiva" but "left for Yeshiva," as in the Jewish school.

My Opinion

If this is the case, and I think so, then I don't think it's "I missed weekend shore" (IMO it doesn't sound like that at all, but this:), I believe it is "I missed weekend shul." then "THIRD time this summer".
http://en.wikipedia.org/wiki/Synagogue