The “month” field in BibTeX entries should be an integer, according to section 2.3.10 in the BibLaTeX manual:
http://ftp.snt.utwente.nl/pub/software/tex/macros/latex/contrib/biblatex/doc/biblatex.pdf
“The bibliography style converts the month [integer] to a language-dependent string as required.”
I would suggest to make this field a drop-down with thirteen options (one blank (default), twelve months) for consistency.

If we want compatibility with BibTeX rather than assume people use BibLaTeX or Biber, the integers should instead be three-letter month abbreviations instead (without quoting braces), as explained in the same section of the BibLaTeX manual and the BibTeX manual:
http://ctan.triasinformatica.nl/biblio/bibtex/base/btxdoc.pdf

Submitted by Maarten Sijm on 29 November 2019 at 20:00