How should the year be capitalized?

Solution 1:

There are only two kinds of documents in current usage that spell out the year -- legal documents, and wedding invitations.

Legal documents normally spell the year in lowercase, such as in this numbing bit of prose from West Virginia:

For any tax imposed under the provisions of this article with respect to any taxable year prior to the first day of January, one thousand nine hundred eighty-three, a resident individual shall be allowed a West Virginia exemption of six hundred dollars for each exemption for which he is entitled to a deduction for the taxable year for federal income tax purposes.

Some legal documents capitalize everything for extra pomp, but it's uncommon:

In testimony whereof, I hereunto subscribe my hand and affix the seal of said Court, at Office, in Nashville, the 6th day of December in the Year One Thousand Nine Hundred Eighty-Four and in the 209th Year of American Independence.

Wedding invitations, as noted, tend to capitalize the first letter only; however, this seems to be the only justification I can find:

The “T” in Two thousand doesn’t have to be capitalized but everyone does it so it would probably look incorrect if it wasn’t and it will look more polished if it is capitalized.

So, in modern usage, it appears that the rules for capitalization are:

• Spell the year out in lowercase.
• Except in wedding invitations, where the first letter is capitalized because everyone does it.

Solution 2:

If it’s any guide, I have a document signed by Elizabeth II dated the fifth day of January, One Thousand Nine Hundred and Seventy-nine.

Solution 3:

I am a person who demands to know the reason for one position over another. Having said that, the following justification is one I prefer to "because that's the way everybody does it"

If the wedding invitation consists of a single sentence, then the regular conventions of capitalization should apply in that sentence, being consistent with Emily Post's view of using all lower case letters for the date except for the proper nouns in the sentence, e.g., Saturday and January.