I've used the command \AtEveryBibitem{\clearfield{month}} to prevent biblatex from printing the month of publication in the bibliography. This has worked fine at least until biblatex version 2.5 and biber version 1.5:
\documentclass{article}
\usepackage[style=authoryear]{biblatex}
\AtEveryBibitem{\clearfield{month}}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@ARTICLE{lennon,
AUTHOR = "John Lennon",
TITLE = "I wrote great music",
JOURNALTITLE = "Journal of Music",
YEAR = "1978",
MONTH = "10",
PAGES = "15--27",
VOLUME = "5"}
\end{filecontents}
\addbibresource{\jobname.bib}
\begin{document}
\nocite{lennon}
\printbibliography
\end{document}

In biblatex version 2.7a and biber version 1.7, however, this command apparently has no effect anymore:

What do I need to do in the new version of biblatex to prevent printing the month of publication?
authoryearlabels incorporate dates as of biblatex 2.6. So if you want to ignore months and days everywhere (including sorting), you need to omit these from the source (as in the post karlkoeller links to). – Audrey Jul 30 '13 at 16:10biblatexfrom printing months in the bibliography, it's reasonable to assume that this would be a desired feature. It would be useful if someone could spell out in plain words and provide a command for how to do that. The answer to the question karlkoeller links to includes things likedatatype=bibtex, which seems tailored to the OP's request, and not a general solution for how to suppress months in the bibliography. – Sverre Jul 30 '13 at 16:33\clearfieldapproach is still relevant to all styles that don't use date labels. – Audrey Jul 30 '13 at 16:48biblatexdocumentation thatbibtexis the default data type. Is it still necessary to specifydatatype=bibtex? I still don't understand why\clearfield{month}has no effect here. The documentation says that "A field cleared this way is treated as undefined by subsequent data commands". – Sverre Jul 30 '13 at 16:57datatypeoption. Citation labels are constructed by the backend, so your have to delete the field at the source. I can totally see how all this is unclear, so I've suggested year-only labels here: https://github.com/plk/biblatex/issues/148 – Audrey Jul 30 '13 at 17:05datatype=bibtexfor sourcemaps in biblatex 2.8+, that will be the default. – PLK Jul 31 '13 at 21:37