The preposition “a” is put before the day, both when talking about the day of the week and of the month, when it introduces a complement to the verb “estar“: Estamos A lunes (in answer to the question ¿A qué (día) estamos?–What day is it?); Estamos A 28 de septiembre (in answer to the question ¿A cuántos estamos?–What day of the month is it?). If the verb ser is used to express the date, it must not be followed by a preposition, but instead the verb in the third person singular: Es lunes; Es 15 de julio (in answer to the question ¿Qué día es hoy?–What is today’s date?). “A” is also used before the day when the day is expressed without an article and as a complement to an expressed or implied verb: Expido el presente certificado A 3 de enero de 1998 (I issue this certificate on January 3, 1998); [Firmado] En Madrid, A 8 de junio de 2000 ([Signed] In Madrid, on June 8, 2000). In all other cases, the day is written without a preposition and preceded by an article: Te llamaré el lunes (I will call you on Tuesday); Comienzo mis vacaciones el 20 de junio (My vacation starts on June 20th).
The preposition “en” goes before the month: Estamos EN mayo (We are in May); or the year, if no reference is made to the month: Nació EN 1978 (He was born in 1978). The use of “en” immediately before the day of the month is archaic and should not be used now: La ley se aprobó EN 3 de mayo (The law was approved on May 3–INCORRECT IN SPANISH); the most common practice in these cases is to use the article: La ley se aprobó el 3 de mayo (The law was approved on May 3–CORRECT IN SPANISH).
The preposition “de” is used between the day and the month and between the month and the year: Murió el 15 DE julio DE 1957 (He died on July 15, 1957). “De” is also used before the name of the month, when stated: Estamos en el mes DE septiembre (We are in the month of September). It is not necessary before years when the year is written out numerically, though it is done at times in literary or formal contexts: Murió en el año (DE) 1974 (He died in 1974).