Patras' carnival is one of the most famous in Greece. The carnival festivities take place each year between mid January to mid February (various dates). It ends with the burning at the stake of the King of Carnival and a big parade of floats and costumed groups.
The carnival's program, venue, lineup information show time / schedule of events, how to get, parking etc. is present in the carnival's link website. See details below, under "Event Related Information" section.