January -- The item for January is ‘0’ = 0 February, March -- The item ... for February or March (the 3rd month), ‘3’ = 3 December -- The item ... for Decembe

January -- The item for January is ‘0’ = 0 February, March -- The item ... for February or March (the 3rd month), ‘3’ = 3 December -- The item ... for December (the 12th month), ’12.’ So these all use fixed values. The `(10-place)+days mod 7` logic works for May, July, September, and November using the previous month's values -- effectively every other month. The vowel trick is likely a nmemonic for not having to remember which months to do this for. The other months are the previous month's `item+days mod 7`, although it is unclear where this interpretation is deduced. -- The "If it begins or ends with a vowel, subtract the number, denoting its place in the year, from 10." text does not apply, so I would have assumed that is was 0 not the calculated item value. Thus, I would have expected it to just use the days in month.