Question

what occurs twice in a week

Answer 1

What occurs twice in the week is the letter E. The answer of the riddle depends on the spelling of the word (week), where only the letter E comes twice.

Answer 2

What occurs twice in a week is the letter e. That is a riddle and the whole riddle is : 

what occurs twice a week,once in a year but never in a day

So, we notice that the alphabet e comes twice in the word week, once in the word year and never found or it's absent in the word day