Cuba's ruling Communist Party opens its congress today (Dec. 17). It occurs every five years and is the most important event in the island's political calendar. Political controversy is most unlikely, and officials say 80 percent of the meeting's time will be spent on the 1981-85 economic plan. No changes at the very top are expected, but some diplomats here believe some of the 112 members of the Central Committee could be dropped for personal failings. The 13 -member Politburo could also be expanded to include several rising politicians who have not been promoted to this level. The congress will review the island's first five-year plan, now near its end, and endorse modest reforms aimed at raising productivity.