The letter "E" appears twice in the word "week" but only once in the word "year." The question is a riddle that plays on expectations.
People can be tricked by focusing on trying to identify an event that happens twice during the span of a week but only once during the span of a year. However, the riddle's answer actually refers to the frequency with which the letter "E" appears in each of the two words. A similar riddle is: "What occurs once in a minute, twice in a moment and never in one thousand years?" The answer is the letter "M."