Answer:
The letter "r"
Explanation:
As you can see, in the literal "middle" of the words "March" and "April", there's the letter "r": March and April.
However, the letter "r" doesn't appear at the start or end of each word, and obviously, you don't see the letter "r" at the beginning and end of a physical month.
Thus, the answer is the letter "r".
Hope this helps!