Arika Okrent has a nice list of holiday (ish) proclitics, including 'tis, 'twas, and the not-terribly-festive y'all:
English likes to stick contractions on the end of words. "They have" becomes "they've," "I will" becomes "I'll," and "do not" becomes "don't." The shortened parts of these words are called enclitics — they are a bit more independent than suffixes, but like suffixes, they attach to the ends of words. English also used to have a number of proclitics — shortened words that attach to the beginning of other words. Most proclitic words are now archaic or obsolete, but every December the neglected proclitics get their revenge, as a holiday avalanche of "'tis" rolls through town.
'Tis, a shortening of "it is," has a Dickensian, Christmasy ring to it. For a time, it was far more common in writing than its counterpart "it's." The final shift from "'tis" to "it's" took place in the middle of the 19th century, when Dickens was writing his novels. That was also when the lyrics to "Deck the Halls" were first published. "'Tis the season" is now so deeply embedded in our linguistic consciousness that the perfectly normal phrase "it's the season" just sounds weird, like Mick Jagger singing "I can't get any satisfaction."
Another fun set are old-timey swear words, such as zounds (from God's wounds), 'struth, and 'sblood.
Clitics are a type of morpheme that is midway between a full word and an affix: they depend on another word but not as tightly as an affix. Just like we can have prefixes and suffixes, we can also have proclitics (before the word they lean on) and enclitics (after the word they lean on). Apparently there are also mesoclitics and endoclitics which are like the infixes of the clitic world, but they're pretty rare.