Adverb? Like "weekends" in "we go out weekends"? Not very adverbial to me. Equinox◑ 14:45, 18 July 2010 (UTC)
This and all its large number of relatives ending in "-s" are apparently the "adverbial" genitive, dative, or accusative. (An instructive example is "once".) When I discovered that there were names for this, I added a few of them. I stopped, but they are real. I have a modern synchronic grammar (CGEL) and one volume of an older diachronic grammar (Curme). Curme states: "In oldest English, nouns in the genitive, dative, and accusative were often used adverbially." One example cited is: "The museum is open Sundays" (or even "Sunday's"). But nowadays (!!!) it is often "on Sundays". Further, Curme states "In colloquial speech it [adverbial genitive] is still common in a few nouns to indicate repeated occurrence, but it is now felt as an accusative plural: 'returning nights to his home.'"
I cannot find a discussion of the frequency usage in CGEL. CGEL does talk extensively of the use of nouns as temporal adjuncts, however. As Brett likes to tell me, I need to make a distinction between function and part of speech. This is a POV (certainly defensible and modern), as most definitions part of speech read like this one from MWOnline: "a traditional class of words distinguished according to the kind of idea denoted and the function performed in a sentence".
I further note Monday#Adverb which covers "He came home Monday." (temporal location) and "He worked all Monday." (duration). Should we treat adverbial use of proper nouns like Monday differently from/than days#Adverb ("The decision came days before the administration was due to impose sanctions")? day does not have such an adverb PoS section. DCDuringTALK 16:42, 18 July 2010 (UTC)