An epithalamion is usually a wedding poem (though it can be a song, a piece of music, or even a painting).
It seemed appropriate to feature epithalamia in the week of a royal wedding that's been received with glee, rejoicing, loving-kindness, ennui and, in some small dark unhappy quarters, contempt.
Ah well. This has probably been the reaction to weddings down the ages, for they've been around a long time. Epithalamia themselves are ancient. The Song of Solomon is often counted as one, and that was written...well, people argue about that, but certainly earlier than 300 BC; and the Greeks were singing epithalamia even before that, usually at the door of the bridal chamber. One epithalamion would be sung as the couple retired, and then another (possibly even less welcome) would wake the couple up the next morning. Epithalamia were generally full of good wishes and hopes for a happy marriage.
The Romans took up the custom, though they tended to sing theirs at the reception after the happy couple had left - and the words tended to be less suitable for sober society.
The tradition stuttered a bit after that, though the French Ronsard and the Italian Metastasio were part of crazes for ephithalamia, and there was a similar English craze that saw John Donne, Ben Jonson and, very notably, Edmund Spencer write wedding poems (Spencer's takes up a whole book).
Nowadays wedding poetry is rare, though, having just come from doing a Google search of 'poems for a wedding' I can say with great feeling not nearly rare enough.
In fact the offers on Google enough to make me wonder if the original custom of singing the epithalamion in private, away from the guests, might really have been the most sensible way of going about it.
Word To Use Today: epithalamion. This is Greek. Epi- means upon, and a thalamos is a bridal chamber. The official plural is ephithalamia, but -mions is also used a lot.