When is Christmas Eve Eve?

Originally, "Christmas Eve" meant the night before Christmas day.

Today we count days from midnight to midnight. That is, we begin each new day at midnight. On the ancient Jewish calendar, the day went from sunset to sunset. That is, sunset marked the beginning of a new day. When Christians borrowed holidays from the Jews, they borrowed this idea of the holiday starting at sunset. (See, e.g., Catholic Doors Ministry presents Christmas Eve)

Somewhere along the line we switched to the Roman practice of starting the day at midnight, but retained the "eve" of some holidays as the night before. So the night before Christmas day, that is, the night of December 24, is Christmas Eve. The night before New Years, that is, December 31, is New Years Eve. (That's the only holidays I can think of where we do this. Nobody talks about "Fourth of July Eve" or "Veterans Day Eve". Maybe there are other examples.)

Many people now use "Christmas Eve" to mean the entire day before Christmas and not just the night. From there it's a short step, I guess, to saying "Christmas Eve Eve" to mean December 23. But this is not accepted practice; it's more of a joke.

Mrs. Claus introduced this terminology in 1987

SANTA: Not so merry. The elves are behind and it is Christmas Eve.

MRS. CLAUS: Yes, last night was Christmas Eve Eve.
And the night before that was Christmas Eve Eve Eve.
And the night before that was Christmas Eve Eve Eve Eve

SANTA: I know dear.

MRS.CLAUS: My favorite is the day after Christmas. That's when I get to say Christmas followed by 364 Eves. Christmas Eve Eve Eve Eve Eve...

(Rough Night at the North Pole, first performed 12 December 1987, Group Repertory Theatre, North Hollywood California)

According to Fowler's Concise Dictionary of Modern English Usage:

Eve means 'the evening or day before' (as in Christmas Eve) and, in figurative use, also means 'the time just before an event' (as in the eve of the election). In the following examples, eve is literal in the first two phrases, is figurative in the third and may be either in the last two:

• on Christmas Eve
• on the eve of Saint Agnes
• on the eve of great developments
• on the eve of a battle
• on the eve of departure.

The meaning in particular cases is often clear from the context.

Christmas eve eve appears to be a colloquial form used to refer to the 23rd of December:

There is a UD entry for Christmas Eve Eve . Its 6k plus up thumbs probably make it worth a look.

• The day before Christmas Eve, 2 days before Christmas.
• Stay away from the malls on Christmas Eve Eve.

• Today is Christmas Eve Eve.

From When The Snow Falls:

• “Today's Christmas Eve eve,” Roberta went on. “It's when Gary and I always celebrated. We never could do Christmas Eve or Christmas Day, as there were always his sisters and brother or his mom, when she was alive, and of course his dad, long ago. We made a pact to always celebrate on the twentythird . . . today.

Ecclesiastically, the eve of a feast was the night before. However, in common use, the eve is the day before.

Christmas Eve Eve isn't a commonly-used term, so has no formal definition but I interpret Christmas Eve as the day before Christmas, and Christmas Eve Eve as the day before that. (Or perhaps the Christmas ere-eve).