Does en fecha ayer mean as of yesterday in Spanish?
No, yesterday in spanish is simply "ayer", "fecha" on the other
hand is date is and calender date
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"En fecha ayer" is sometimes used in South American, but it´s
not Standard Spanish. The correct form is "En fecha de ayer".
Likewise, you can also say "en fecha 1 de junio de 1990" or "en
fecha de 1 de junio de 1990". If a name follows "en fecha" the
preposition "de" ("of") is mandatory.