Spanish term
United States
Your brand's voice in good hands
Native in: English (Variant: US) 
Works in: Spanish to English
a un día del mes de...
I think it is either "on the first of [Month]" or simply "on a day in [Month]"
On the certificate (from Honduras) the context is simply as follows:
[CITY]
a un día del mes de diciembre de dos mil diecisiete
[signatures]
In legislation, I find examples like:
"Aprobado el Proyecto de Ley por la Honorable Cámara de Diputados, a un día del mes de noviembre del año dos mil uno, y por la Honorable Cámara de Senadores, a once días del mes de abril del año dos mil dos, quedando sancionado el mismo, de conformidad con lo dispuesto en el Artículo 207, numeral 1 de la Constitución Nacional."
| 4 +7 | on the first day of... |
Helena Chavarria
|
Jul 4, 2018 21:06: Helena Chavarria changed "Level" from "Non-PRO" to "PRO"
PRO (3): AllegroTrans, Robert Forstag, Helena Chavarria
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Proposed translations
Spain
Striving for excellence
Native in: English (Variant: British) 
Works in: Spanish to English, Catalan to English
on the first day of...
In Lima, on the third day of the month of November of the year two thousand and five.
https://www.linguee.es/espanol-ingles/traduccion/a tres días...
I think the reference you've provided at the end of your question says it all: 'a once días del mes de abril del año', which would be the eleventh day of April (11 April).
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Note added at 19 mins (2018-07-03 18:11:37 GMT)
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En la Ciudad de San Luis, a doce días del mes de Agosto del año dos mil ocho, reunidos en su Sala de Acuerdos los Señores Magistrados de la EXCMA. CAMARA CIVIL, COMERCIAL, MINAS Y LABORAL DE LA PRIMERA CIRCUNSCRIPCIÓN JUDICIAL DE LA PROVINCIA DE SAN LUIS.
http://www.justiciasanluis.gov.ar/wp-content/uploads/Documen...
se otorga el presente certificado, a nueve días del mes de septiembre
This certificate is awarded the ninth of September.
https://forum.wordreference.com/threads/se-otorga-el-present...
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Note added at 28 mins (2018-07-03 18:20:01 GMT)
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On the first page you can read: '...Honorable Cámara de Diputados en sesión ordinaria de fecha 9 de agosto de 2017' and on the third page it says: 'DADA EN LA SALA DE SESIONES DE LA HONORABLE CÁMARA DE DIPUTADOS DE LA NACiÓN, A NUEVE DíAS DEL MES DE AGOSTO DEL AÑO DOS MIL DIECISIETE.
The same thing happens on pages 4 and 6.
http://odd.senado.gov.py/archivos/file/MHCD Nro 2347.pdf
| I am so late to respond, but thank you Helena. I am still confused as to why they don't write "el primero de [mes]" in that case but it must just vary. |
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