"you never write the definite article before the date, but you pronounce it."
There a few factors that come into play with use of "the" in the date.
Firstly; it is more common for American people to miss out "the" than British people (older generation at least).
Secondly; American people often use the form December the third (December 3rd), whilst UK English tends to use the form the third of December (the 3rd of December).
Thirdly; the context for example if the date is specific rather than general. The Fourth of July. The twenty fifth of December. Where the date has a special meaning.
As to the Statement you never write the definite article this is not exactly correct.
In UK English Cambridge English Dictionary fifth; ordinal number; 5th written as a word:
Tomorrow is the fifth (of September).
In US English fifth; adjective, adverb, noun; (a person or thing) coming immediately after the fourth and before all others:
Tomorrow is the fifth of September.
So even in US English the use of the written form of the date makes it a specific event and "the" should be used. Whilst I suggest in the numerical form it can be omitted.
Tomorrow is, 5th of September - Tomorrow is September 5th.