Continuing on the topic of writing dates in Spanish correctly, regarding the use of the article “el” (which includes the contraction “del“) before the expression of years, these are the proper protocols:
a) For years between 1 and 1100, it is common to use the article, at least in spoken Spanish: Los árabes invadieron la Península en EL 711 (“The Arabs invaded the Peninsula in 711”). There are numerous examples where the article is omitted in written texts: Ya en 206 a. de J. C. tiene lugar la fundación de Itálica (“Italica was founded in 206 BCE”).
b)For years between 1101 and 1999, expressions without the article are much more common: Los Reyes Católicos conquistaron Granada en 1492 (“The Catholic Kings conquered Granada in 1492”), though examples that include the article can still be found: Nací en EL 1964 (“I was born in 1964”). If the year is abbreviated, with the first two numbers not included, it is mandatory to include the article: En EL 92 se celebraron las Olimpiadas de Barcelona (“The Olympics were held in Barcelona in 92”).
c) Starting in 2000, the change of the thousands column brought about the rebirth of the article: Fui al Caribe en el verano DEL 2000 (“I went to the Caribbean in the summer of 2000”) or La autovía estará terminada en EL 2010 (“The highway will be completed in 2010”).
However, when putting dates in letters and documents, the differences listed above are not as noticeable and writing without the article: 14 de marzo de 1420 (“March 14, 1420”), is preferred, as has been the case since the Middle Ages. With this in mind, it is also recommended to maintain that style in letters and documents from the year 2000 and beyond: 4 de marzo de 2000 (“March 4, 2000”). Naturally, if the word “año” is explicitly included, you must put the article: 5 de mayo DEL año 2000 (“May 5, 2000”).
Source: Real Academia Española