At the blog Apostrophe Catastrophe, every post consists of a picture of some public piece of text with a misplaced or missing apostrophe, with commentary by the blogger, a Bostonian named Becky. The picture for October 31, 2009 showed a poster advertising a Halloween party at a Boston bar. Across the top, the poster read, "Come Rock With the Jock's." Becky poked fun at the inappropriate apostrophe in "Jock's," wondering where "the jock" was. Had I been the one making snarky commentary, I would have first asked, "Rock with the jock's what?" But enough about the jock and his mysterious possession. Lower down on the poster were the words Halloween Party, and Becky had nothing to say about that missing apostrophe in Halloween. In fact, I don't think I've ever seen or heard a punctuation peever complain about this now nearly universal misspelling of Hallowe'en, even though the changeover happened recently enough (see the Google Ngram view) to have inspired at least some grousing about it. 

As most histories of Halloween will tell you, Hallowe'en (or Halloween) is a shortened version of All-Hallow(s)-Eve, but how and why did eve turn into e'en? For that matter, what is a hallow? Why did the all get dropped?

I'll get the last question out of the way first: I don't know why the all disappeared from Halloween. The citations in the Oxford English Dictionary have it with the all from the earliest one in 1556 to one in 1616 from Shakespeare (Allhallond-Eue, in Measure for Measure). From the 1700s onward, it's Hallowe'en.

At least until Harry Potter and the Deathly Hallows arrived, the closest word to hallows heard in present-day English was the verb hallow "to make holy", usually in the form of the past participle hallowed. Christians are familiar with it from the first sentence of the Lord's Prayer, in hallowed be thy name; other than that, it occurs most often in hallowed ground(s) or hallowed hall(s), according to the Corpus of Contemporary American English. In fact, hallow and holy come from the same root. In Old English, holy was spelled halig; with the verb-creating suffix ‑ian it produced halgian "to make holy", which underwent several sound changes over the centuries to end up as hallow. The ‑ian suffix is the closest Old English comes to being able to "verb a noun" without changing it. I've translated halgian as "to make holy", since we can't just talk about "holying" something in present-day English, but still,  "to holy" or "to holify" gives a better sense of having the meaning of "make holy" encapsulated in a single verb.

So much for hallow the verb; what about hallow the noun? J.K. Rowling used it to mean a sacred object, but that isn't its original meaning. It comes from halga, which as the masculine noun form of the adjective halig meant "holy person". In other words, hallow is a native English word for saint. According to the Oxford English Dictionary, the meaning of the plural hallows was extended to mean relics or temples of the saints, and from there, Rowling went a step further to give it her desired meaning.

Moving on to eve and e'en, both are shortened versions of even, an archaic word for "evening". Even itself might seem to be a clipped version of evening (likewise morn and morning), and cultural historian David Skal even writes, "The word Halloween derives from the Middle English hallowen ... and the progressive contracting of evening to even to e'en." He's wrong here: Evening came later. The Old English word for "evening" was ǽfen or éfen, and the same verbal suffix –ian that turned halig into the verb halgian turned ǽfen into the verb ǽfnian "to become evening". The gerund form aefnung is what comes down to us as evening. (With morning, according to the OED, the process was more direct: the –ing was simply attached directly to morn by analogy with evening.)

Also according to the OED, the final n disappeared from even to produce eve in the same way that it disappeared from maiden to produce maid, and even from an earlier form of morn to produce what would ultimately become morrow. The other shortened form of even is an example of what linguists call lenition, a process in which consonants that take more of an effort to produce are replaced with phonetically similar ones that are easier. In the pre-Old English period, the precursor of ǽfen probably had a b instead of the f. (The word in modern German is abend.) Replacing a plosive consonant (such as b) with a fricative (such as f) is a typical stage of lenition. Also typical is for a fricative to drop out entirely, precisely the change that later gets us from even to e'en.

With both eve and e'en to choose from, why did we end up with Hallowe'en instead of Halloweve? (Or conversely, why did we end up with Christmas Eve and New Year's Eve instead of Christmase'en and New Year's E'en?) Robert Burns's 1786 poem "Hallowe'en" seems to have given e'en the decisive edge.

The missing apostrophe is a sign that the word Halloween has become, for many people, monomorphemic—a word that you can't break down into meaningful parts. For many speakers, even those who know the word hallow, it's no more a component of Halloween than sham and poo are components of shampoo (though it's fun to imagine they are)Further evidence of monomorphemic Halloween is the fact that you can take just part the -een for use in blended words with no loss of meaning. For example, the hypothetical word Christmaseen mentioned earlier has actually been coined independently by a few speakers, but it doesn't mean "Christmas Eve"; it means the imaginary holiday that stores must be preparing for when they put out their Christmas merchandise before they've cleared away their Halloween items. Of course, you could also make the same argument about the blend Chrismukkah, in which just Chris stands in for the concept of Christmas. Still, many more people are aware that Christmas has two parts than are aware that Halloween does. Bumper stickers and billboards remind us that Christmas is dimorphemic, and encourage us to keep it that way. They don't urge us to "Keep the hallow in Halloween."