These two examples mean the same thing. However, on certain occasions they can mean differently. For example, "At the table" or "on the table" means different things: one refers to beside the table, the other refers to on top of the table.
The reason "occasion" is not affected by changes of "at" or "on" is because it refers to an occurance in time, whereas geographical locations or objects are affected by the difference of prepositions. i.e. "table", which is a geographical object, is affected when "at" is changed to "on", or "on" changed to "at."