Hi - I remember when I was at school in England (a very long time ago) having explained the use of "th" when writing dates and the need not to use "of" as well.
As part of a team correcting business letter writing errors (in Australia), I need to explain to users why they should write:
Tuesday 15th August
Tuesday the 15th of August or Tuesday 15th of August
Whilst users are receptive to following "the rules", I cannot find them on the intranet to refer them to.
There are many conventions, regional differences and personal preferences in this area, which is perhaps why there are no rules - just suggestions of alternatives and comments on their suitability for different situations and purposes.