Possibly a Bulgarian loanword, compare dialectal крачун (kračun, “winter solstice; a Slavic pagan holiday celebrated at the winter solstice”), also compare Macedonian dialectal крачун (kračun, “Christmas”), Slovak Kračun (“winter solstice feast”) (dialectal, "Christmas"). From Proto-Slavic *korčiti (“to step”) - the Sun in becoming "bigger". It's possible that these languages received the word, at least in this sense, from Romanian, however. See also Hungarian karácsony (“Christmas”), which is also related.
Or it might alternatively be from Latin creātiō, creātiōnem, with the meaning derived from that of the creation or birth of a child, e.g. Jesus' birth on Christmas. Compare the archaic meaning of Spanish criazón ("person or child living in a house under the authority of another"), of the same origin (compare crío and the verb criar in Spanish); compare also Sardinian criatzione, with meanings related to child. The Romanian word had an older, archaic meaning of "birth" in church or religious usage, and is also used for the holy image of Christ's birth. Other less likely etymologies proposed include Latin Christī followed by an uncertain second root, such as Christī iēiūnium (compare also Albanian Kërshëndella from Christī nātāle), Latin (in)carnatiōnem, crastinum (“tomorrow, the morrow”), or calātiō, calātiōnem (“calling, summoning”) (this one is supported by a number of linguists). Compare also Aromanian Crãciun, which indicates a probable Latin origin for the word, and compare the Romanian suffix -ciune. 
Crăciun n (plural Crăciunuri)
Crăciun m (genitive/dative lui Crăciun)
- A surname.