In English the day as part of data specification is expressed in an ordinal number, although it is not always written as a ordinal number: 7 february (the seventh of february)
In Portuguese and in any other roman language the days are expressed with cardinal numbers. Only the first day of a month is expressed with a ordinal number. (the 1st of january <=> 1.º de janeiro <=> primeiro de janeiro, the 1st of may <=> 1.º de maio <=> primeiro de maio etc.. )
From a purely logical point of view, a completely irrelevant perspective when it comes to analyzing linguistic structures, the ordinal numbers would be more logical, because in the data we describe the position of an element inside an order, the 5th of june is before the 20th of june, we are not in interested in the amount of days, this is expressed by a cardinal number. The Portuguese don't say either ~the two from the left, but the seconde from the left, o segundo da esquerda. The phenomenon that the cardinal numbers are used instead of the much more logical ordinal number is due to the fact that the ordinal number in roman languages are difficult to form, see 25.1. The 28th of december written with an ordinal number would be vigésimo oitavo de decembro. That would be too long and to difficult.
The preposition de is semantically motivated, it is a genetivus possessivus. The same role plays the preposition of in English. The difference is that in English it is not written. The preposition is a hint that originally the ordinal numbers where used in this context. We write 25.07.2000 but sometimes we spell it twenty-fifth of July two thousand. The genetivus possessivus describes the owner of something and the 10th of january belongs to january.
The data at the beginning of a letter has therefore this form.
the day in a cardinal number
name of the month
Lisboa quinze de março de dois mil e doze
If the data starts with the day and if there is a preposition in English there is an article.
We have to finish this project until the second of August 2006.
Er hat noch bis zum (zu dem) 13. Februar 2012 Zeit.
no article when it starts with the month: I don't know anything about the situation before June the sixteenth 1999.
Concerning Portuguese there is no "oficial" statement available, at least this author didn't find one. If we analyze the empirical data we come to the conclusion that the article is not compulsory in this context, after a preposition, but there is a preference for the option with article. An exception are the preposition that end with de, antes de, depois de, por causa de etc.. In this case in general there is an article. This can be explained by the fact that the article is merged with the preposition and therefore it is not necessary to introduce it as an isolated element.
data specification with preposition
Ele exerceu essa função até o 10 de abril de 1937.
He worked in this position until the 10th april 1937.
Só depois do 10 de Junho comecei a pensar nisso.
It is only after the 10th of june that I started to think about it.
O homem encontra-se desaparecido desde o dia 16 de junho de 2017.
The man disappeared the 16 july 2017.
Esse processo vem se agravando desde o 11 de setembro de 2001.
This process aggravated more and more since 11 september 2017.
A política não será a mesma depois do 14 de março de 2017.
After the 14th of march 2017 the police won't be the same anymore.
No dia 2 de fevereiro, os militares egípcios exigiram o fim das manifestações em massa.
The second of februrary the military in Egypt demanded a stop of the mass rallies.
O melhor é que a recolhas antes do 10 de outubro.
The best time to pick them is before the 10th of october.
data specification without prepostion
Até 2 de fevereiro de 2017 encontram-se abertas as inscrições.
Until the second of februrary one can enrol.
Em 17 de fevereiro de 1600, Giordano Bruno foi queimado vivo no Campo dei Fiori.
The 17th of february Giordano Bruno was burned alive on the Campo dei Fiori.