Can U Tell Me........ What occurs! Once in a Year, Once in a Day, But never in a Week ?

Can U Tell Me?

What occurs!

Once in a Year,
Once in a Day,
But never in a Week ?

In Hindi: वो क्या है जो साल में एक बार आता है, दिन में भी एक बार आता है पर सप्ताह में नहीं आता।

Answer: alphabet A

ex: yeAr, dAy, week