Just to go along with what Andy and Jeff have stated...
I have almost 3000 66/67 trim tag's data. I've tried every conceivable way to "correlate" and build week letter A...E with days of a given month and have yet to come up with anything that's even close - especially with "E" week codes. Just a couple of examples:
1966 August (for 1967 model year) - Kansas City and Fremont
1966 September (for 1967 model year) - Atlanta, Baltimore, and Fremont
1966 October (for 1967 model year) - Fremont
1966 November (for 1967 model year) - Baltimore, Kansas City, and Fremont
1967 May - Kansas City and Framingham
1967 June - Kansas City, Baltimore, and Fremont
So just for the Fremont plant, they used "E" coded trim tags for 4 straight months, Kansas City for 2 straight months.
1966 March - Fremont, Flint, and Kansas City
1966 April - Fremont and Flint
I don't compile data for other years but have noticed the same holds true for any plant, any year. The week letters still do not correlate with any calendar days when you look at two or three months from the same plant.
On a side note as soon as I finish up with my 1970 Chevelle SS In Detail
book I'm going to get back to my 64/65 Reference CD I'm working on.