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? Should I join Amazon hiring event next week if I am going to be ready to solve algo-questions only next month (now only 50% chance) or wait for now?