A very badly mangled puzzle.
It usually depends on a precise formulation of the puzzle and tends to be the letter 'a' when asked like "What comes once in every year, but four times in an average day?"
So it is one A in 'every yeAr' and four A in 'An AvergAge dAy'.
Note that this should be phrased as 'every year', not 'a year' to work.
letter e
The answer to this riddle is the letter "e." It appears once in the word "year," twice in "month," four times in "week," and six times in "day."
The letter e
The letter "E."
fortnight
It means that you had your period twice. You're supposed to menstruate around every month.
The letter a
Two (2) times in a month.
Twice a month means that something happens two times in one month, usually on a regular schedule.
vowel
Once or twice a month. You can do it beginning and end of each month or twice a month if it smells really bad.
Sleep.