Originally posted by: Paperdoc
To be honest, I don't think there is a grammar rule to cover this.
I think there is. Look up the definition of "at" in several dictionaries. The word "at" is generally used (for this purpose) when something can be measured on a scale. Think of a clock, spedometer, thermometer, etc. All are ordered scales and thus you can specifiy where in that order that something is located.
The alarm is set at 7 am. (7 am comes on a scale after 6 am and before 8 am).
The car is travelling at 60 mph. (60 mph is on a spedometer scale after 59 mph and before 61 mph).
The heater is set at 68°F. (68°F is on a temperature scale after 67°F and before 69°F).
The meeting is at room 73. (room 73 is after room 72 and before room 74).
Now condider this sentence. "I purchased my car at Friday". Friday is not clearly measurable on a scale. Was it Friday June 6, 2008 which is after June 5, 2008? Or was it Friday May 30, 2008 which is before June 5, 2008? The sentence isn't clear, there is no scale for "Friday". Thus, we don't use the word "at".
Also, the word "at" is less exact that the word "on". If you say "I am on my house", you literally are situated on top of your roof. But, if you are "at your house", then you could be inside watching TV, outside mowing the lawn, or sitting atop the roof. Thus, if you are inexact, you usually use "at".