Epithalamium: From the Greek epi ("at") and thalamos ("nuptial chamber").  A celebratory song or poem, often in sonnet form, in honor of a bride or groom or both, usually praising their virtues, describing the events of the wedding day, and wishing them good fortune....

ODLIS (Online Dictionary for Library and Information Science)

Even among those few folks who still collect printed books of poetry, it's hard to find many who focus on the epithalamium (pl. epithalamia), one of the most ancient of all types of poetry in the Western world. Basically a type of ode, the epithalamium counts among its better known practitioners such ancient worthies as Sappho, Pindar and Catullus; Renaissance masters such as Ronsard, Donne and Edmund Spenser; and modern poets such as Gerald Manley Hopkins and A. E. Housman.  (The examples below are via the Bayerische StaatsBibliothek and the Open Library, respectively:)

It is in fact to Sappho that the earliest known literary epithalamia (ca. 600 BCE) usually are attributed:

Happy bridegroom, Hesper brings

All desired and timely things.

All whom morning sends to roam,

Hesper loves to lead them home.

Home return who him behold,

Child to mother, sheep to fold,

Bird to nest from wandering wide:

Happy bridegroom, seek your bride.

Theocritus (inventor of the pastoral) is, however, the more famous author of epithalamia among the ancient Greeks (the example following is from his The Epithalamy of Helen):

...What Bridegroom! dear Bridegroom! thus early abed and asleep?
Wast born a man of sluggardy, or is thy pillow sweet to thee,
Or ere thou cam’st to bed maybe didst drink a little deep?
If thou wert so fain to sleep betimes, ‘twere better sleep alone,
And leave a maid with maids to play by a fond mother’s side till dawn of day,
Sith for the morrow and its morn, for this and all the years unborn,
This sweet bride is thine to own....

The great Latin poet Catullus (whose poems are known from medieval copies of a single manuscript) accounts for the earliest extant examples of epithalamia in that ancient tongue (ca. 84-54 BCE):

...And, roused by day of joyful cheer,


Carolling nuptial lays and chaunts


With voice as silver ringing clear,


Beat ground with feet, while brandisht flaunts


Thy hand the piney torch.

For Vinia comes by Manlius woo'd,


As Venus on th' Idalian crest,


Before the Phrygian judge she stood


And now with blessed omens blest,


The maid is here to wed....

Edmund Spenser, best known to most book collectors for his great unfinished work The Fairie Queen, produced what generally is conceded to be the best example of the epithalamium in English (he penned it for his own, second, marriage):

...let them make great store of bridale poses,

And let them eeke bring store of other flowers

To deck the bridale bowers:

And let the ground whereas her foot shall tread,

For feare the stones her tender foot should wrong

Be strewed with fragrant flowers all along....

Even folks who don't collect the printed book at all often are familiar (usually unknowingly) with this type of poetry, since the Christian Bible includes an example (until at least the Renaissance, the Song of Solomon was traditionally interpreted as an epithalamium celebrating the marriage between God and the soul and/or the church):

The song of songs, which [is] Solomon's.

Let him kiss me with the kisses of his mouth: for thy love [is] better than wine.

Because of the savour of thy good ointments thy name [is as] ointment poured forth, therefore do the virgins love thee.

Draw me, we will run after thee: the king hath brought me into his chambers: we will be glad and rejoice in thee, we will remember thy love more than wine: the upright love thee....

(This Biblical masterpiece has attracted the attention of many fine presses over the centuries.  The specimen below was produced by the Circle Press:)

Understandably, many modern critics do not necessarily agree with including the Song of Solomon as an example, inasmuch as epithalamia originally were recited to recently married couples by friends standing directly outside (until the Middle Ages, often inside!) the connubial bed-chamber.

Because definitions have become a bit looser over the centuries, it is not uncommon nowadays to find that such poems often are not addressed to a specific couple at all (cf. this recent example by Matthew Rohrer).  And lest one think that all epithalamia are genteel affairs, the example below (penned by 17th century English poet Sir John Suckling) should disabuse one of such notions...

...The maid—and thereby hangs a tale;

For such a maid no Whitson-ale

             Could ever yet produce:

No grape, that's kindly ripe, could be

So round, so plump, so soft as she,

             Nor half so full of juice.

Her finger was so small, the ring

Would not stay on, which they did bring;

             It was too wide a peck:

And to say truth (for out it must)

It lookt like a great collar (just)

             About our young colt's neck....