All-time Month lists can be accessed via Targets.
This will show what you have NOT seen: I have seen 148 species in September but, of ALL the species seen in September by everyone, I am missing 70 species.
To see what you HAVE seen, click the blue hyperlink next to the species total (i.e., the 148 in below example). This will take you to a page that shows ALL the species you have ever seen in September. (photo not included in this blog).
(Thank you, Marshal Iliff!)