Thanks very much, Aleister, for the helpful information. (I'm always happier knowing where and how to tweak.)
I found the syntax and changed it. For other users who may be curious about the same issue, here are my findings:
I changed syntax to
in three of the theme files (theme adapted from Wuhan):
But the result is not ideal. The date format in the sidebar is now too verbose to fit in the sidebar:
March 27, 2006–April 3, 2006
March 13, 2006–March 19, 2006
March 6, 2006–March 12, 2006
Doubtless I could change that, but I also see that the archive page title for any given week nows reads a generic “Archive for 2006”.
I also wonder what would happen to permalinks, which are famously finicky. So plain ole monthly listings for archives is best for me, I think.