BibLaTeX manual says:
If there is no date field in an entry, biblatex will also consider the fields year and month
for backwards compatibility with traditional BibTeX.
Where can I find the month field (present in BibLaTeX source) in entry editor? It should be in “Deprecated fields” or am I wrong?
Yes, if there is a month = {...} field in the source, it should show up in the entry editor. Could you please check out the development version as I remember that there was a recently fixed bug regarding deprecated fields. If this problem persist in the dev version, please open a new bug report. Thanks.
Follow-up here: https://github.com/JabRef/jabref/issues/2233