The days of the week in English, such as Monday, are sometimes pronounced with a final -[deɪ] and sometimes with a final -[di]. For example, Merriam-Webster gives Monday as \ˈmən-(ˌ)dā, -dē\ and Wiktionary gives /ˈmʌn.deɪ/, /ˈmʌn.di/. This is evidently true for both British and American English.
My question concerns the distribution of this variation. Is it dialectical? If so, what dialects exhibit which form? Is it based on register? Stephen Fry says in Moab is My Washpot that it's an example of U vs non-U: "A gentleman does not pronounce Monday as Monday, but as Mundy". This wouldn't explain the variation in the US, though. On the other hand, the two forms do not seem to be in free variation.
I'm not aware of any of any English vowel mergers that would explain this. Is there a DAY-DEE merger? A rule for final [eɪ]-raising?
There's lots of anecdotal information on this, like this English.Stackexchange question, which is interesting enough, but I'm curious if anyone knows of scholarly work on the subject.
EDIT: I have found one scholarly work on the subject, K. Wheatley in American Speech, Vol 9 No 1, Feb 1934, pp 36-45, "Southern Standards". Author writes:
Yesterday, Monday, Tuesday, etc., always have [i] in the final syllable in Southern speech while [ei] is often heard in these words in the linguistic West.
It is not clear what dialects she means by "Southern" and "Western" other than that these are American dialects.