OCCASIONAL, adj. Afflicting us with greater or less frequency. That, however, is not the sense in which the word is used in the phrase "occasional verses," which are verses written for an "occasion," such as an anniversary, a celebration or other event. True, they afflict us a little worse than other sorts of verse, but their name has no reference to irregular recurrence.
2009 Update: At a frequency one tenth as common as a habit and ten times a lucky break, describing both. For example, OCCASIONAL is spelled "occassional" or "ocassional" occasionally.