In camel-casing, a capital letter indicates the start of a new word. Therefore this is clearly "a typical day" while "atypical day" would be "atypicalDay".

wouldnt't atypical day be atypicalDay and a typical day be aTypicalDay? following camel case nomenclature the function name seems to suggest a typical day and is aptly named.

I would interpret the former. Presumably the latter would be "atypicalDay"