For my private investigator friends. Can you answer the riddle?

For my private investigator friends. Can you answer the riddle?

Originally shared by Escape Room Cozumel

I am the first on Earth, 
the second in Heaven 
and in the middle of the sea. 

Although I appear two times in a week, 
you can only see me once in a year.
 
What am I?

#riddle   #puzzle   #cozumel