Some of the songs we usually think of as happy Christmas songs are, at least in their original versions, rather sad or depressing.

E.g., "Have Yourself A Merry Little Christmas" was sung tearfully by Judy Garland in the film in which it was introduced.   Later, Bing Crosby, I think, changed the lines "Someday soon we all will be together, if the fates allow. Until then we'll have to muddle through somehow, so Have Yourself ..."   to "Through the years we all will be together, if the fates allow. Hang the shining star upon the highest bow, and Have Yourself ... "   I.e., a very wistful song of yearning and missing loved ones was transformed into a rather saccharine song about cosy holidays with family.

(This song evokes even more bittersweet feelings for me because in the movie "The Victors" the song was played as soundtrack for an execution of a deserter in WWII on Christmas Eve.)


Also, Longfellow's poem "I Heard The Bells On Christmas Day" was not the optimistic-sounding lyric that we hear in the song version, which was published about 10 years later.
In the original poem, inspired on Christmas day 1860 (IIRC), the verse about "Till ringing, singing on its way, the world revolved from night to day ... " came right after the verse that goes "I thought, how as the day had come, the bellfried of all Christendom had pealed along th'unbroken song of "Peace on earth, good will to men."
THEN came the two verses that clearly place the song in the US Civil War:

I Heard The Bells On Christmas Day

Here's Longfellow's original poem:

CHRISTMAS BELLS
(Henry Wadsworth Longfellow, 1863)

I heard the bells on Christmas Day
Their old, familiar carols play,
    And wild and sweet
    The words repeat
Of peace on earth, good-will to men!

And thought how, as the day had come,
The belfries of all Christendom
    Had rolled along
    The unbroken song
Of peace on earth, good-will to men!

Till, ringing, singing on its way,
The world revolved from night to day,
    A voice, a chime,
    A chant sublime
Of peace on earth, good-will to men!

Then from each black, accursed mouth
The cannon thundered in the South,
    And with the sound
    The carols drowned
Of peace on earth, good-will to men!

It was as if an earthquake rent
The hearth-stones of a continent,
    And made forlorn
    The households born
Of peace on earth, good-will to men!

And in despair I bowed my head;
"There is no peace on earth," I said:
    "For hate is strong,
    And mocks the song
Of peace on earth, good-will to men!"

Then pealed the bells more loud and deep:
"God is not dead; nor doth he sleep!
    The Wrong shall fail,
    The Right prevail,
With peace on earth, good-will to men!"

Although the last verse is hopeful, the poem makes the reality of war very evident. About 10 years after Longfellow's poem was published, someone rearranged the verses, removed the explicitly war-related verses, and set the song to music.   (At least 2 different tunes have been used - 3 I think.)



Other, even more depressing "Christmas" songs include:

Mary Mild (or The Bitter Withy) - although they deal with Jesus as a young boy, they are often sung at Christmas time

El Santo Niño (The Holy Child)

Green Christmas - Stan Freberg - Stan took on the commercialization of Christmas so poingnantly and pointedly back in the 1950s that the song was pretty much blacklisted by radio stations