In normal speech the final syllable generally has the weakened pronunciation [di], but in careful speech it can also be pronounced [dei]. Another variant pronunciation for "Wednesday" is [ˈwɛdn̩zdi] or [ˈwɛdn̩zdei] (where [n̩] is a syllabic [n]).

We rarely pronounce the last syllable of the days of the week in the same way as the word 'day' /dei/. The important point here is that it is an issue of stress. The stress falls on the first syllable of these words. The last syllable is therefore weak and unstressed, and not pronounced with the same clarity or force as the stressed syllable. For this reason, it is usually pronounced as /i/.

Both variants are used, but the first is considered more proper.

Sunday [ 'sΛndei ] Monday [ 'mΛndei ] Tuesday [ 'tju:zdey ] Wednesday [ 'wenzdei ] Thursday [ 'θə:zdei ] Friday [ 'fraidei ] Saturday [ 'sætədei ]

Both pronunciations are acceptable. Neither will be misunderstood. I think that native speakers MEAN to say -dei, that is to pronounce the full syllable just like the word "day." We may even think we are saying -dei. The reason people say -di is that it is what naturally happens when speaking quickly. If you listen to songs intended to teach the days of the week to children, e.g. https://www.youtube.com/watch?v=LIQsyHoLudQ the last syllable is always pronounced fully, -day. On the other hand, I see that the American Heritage dictionary, https://ahdictionary.com/word/search.html?q=Sunday&submit.x=44&submit.y=26 lists both pronunciations but gives the -dē form as more frequent: sŭn'dē, -dā′ There's an old nursery rhyme that seems to require the -dē for as a rhyme, although it IS old... and I don't know for sure how "Grundy" would have been pronounced: Solomon Grundy, Born on a Monday, Christened on a Tuesday, Married on a Wednesday, Ill on Thursday, Worse on Friday, Died on Saturday, Buried on Sunday. That was the end, Of Solomon Grundy