grammatical number - Dos and donts or dos and donts . . . The following nGram suggests that dos and donts, do's and don'ts, and do's and don't's are all used, but it appears that do's and don'ts takes the cake A quick search suggested that capitalizing all but the s is also fairly popular:
Where does the phrase fair dos dues doos does come from? The other quotations use these spellings: Fair do's, fair do's, fair dos, Fair do, fair do's The first of these is from 1941: 1941 L A G Strong Bay 168 Come on, Doctor Fair do's The spellings are usually a variation of the noun do (under which the phrase is listed in the OED), because a do is something done, a dealing or treatment