Language Log has an interesting post on the metaphorical underpinnings of pronoun usage for time. Mark Liberman sets it up:
Last night at dinner, several Americans and a Canadian got into a discussion with an Irishman and an Australian about weekends. Since all of the participants were linguists, the discussion centered on prepositions: Were we having dinner on a weekend in February or at a weekend in February? The North Americans voted for "on", a choice that the Irishman found preposterous. "A weekend," he observed, "is not a surface."But he was forced to admit that the appropriate usage is on Saturday, not at Saturday, and on Sunday, not at Sunday. "So," countered one of the Americans, "Saturday is a surface, and Sunday is a surface, but their combination is not a surface?"
At the moment the post has attracted 72 comments, though I've only read the first dozen or so. I particularly like this suggestion by Lllessur Llihgdots:
I think the divisions are more or less correct but it might be more coherent to formulate it in terms of 1D, 2D, or 3D perspectives ("at", "on" and "in", respectively) as it seems a bit less ad hoc. (I see Coby Lubliner has also mentioned in a previous comment.) The temporal sense echoes the underlying physical distinctions in the originals, e.g., "at the corner", "on the corner","in the corner"."At" marks a 1D point in physical terms, e.g., "at Shinjuku Station" when viewed from a suitably abstracted view, while "in Shinjuku Station" when zoomed in. This extends to time with uses like “at 9”, “at the age of 20”, “at midnight” etc. The notable exception might be considered “at night”, which doesn’t seem to indicate a precise time though this may be a historical anomaly and perhaps a truncated version of “nightfall”. In the temporal extension of the underlying metaphor, "at" is used in reference to a point in a timeline. In the case of UK usage, "at" is used true to the literal meaning of "week-end" as being the point at the end of the week timeline."On" might be considered 2D and used in a planar sense. Since the notion of days is dependent on a calendar and a place-holder (at least figuratively), it would make sense to use "on" in this context. This usage is most likely reinforced by the physical spaces provided on calendars and in diaries. The US usage seems a correction towards the dominant metaphor of counter days rather than the timeline model sense rubbed from the word through use. This is probably strengthened by the "content" metaphor of the weekend days and the use of "on" in those senses, e.g., "On Human Nature", etc. (I would suggest that the content and “continuous motion” uses of “on” with words like trip, journey, vacation, picnic, mission, etc. are extensions of the underlying planar metaphor.)"In", in keeping with the underlying physical metaphor, is 3D and so describes times whose relations are surrounding the reference point.
John Lawler offers this:
Way back in the 1960s, when I taught ESL, I used to tell students that the size of the time period controlled the preposition: big periods used in: in a week/ten years/the next century, small periods used at: at 2:53/noon, and periods in between, especially named ones, used on: on Wednesday/the weekend/. This was serviceable, if weird.Later I learned, via the late Chuck Fillmore's Deixis Lectures, that there is a lot more involved.