Help me Out!

How often do we task our brain? Well don't bother thinking too much about that cause I will make you do that now.
I have being all this riddle for months but I know I have smart followers so help me out.

I am the first on Earth, the second in Heaven. I appear two times in a week you can only see me once in a year. Although I am in the middle of the sea, l am not found in month, but l am in February, June, September, October, November & December. What am I?