http://www.nytimes.com/2009/12/03/business/smallbusiness/03marketplaces.html?pagewanted=all"Selling through auction requires more strategy. Sellers generally want their auction listings to expire at
the time of week when the listed item tends to sell best. Mr. McGrath, for instance, found his pepper mills and other kitchen gadgets sold best on Sunday and Monday evenings."
According to dictionaries, "week" is very much countable. So, did NY Times make a mistake, or am I missing something?