It is located in Kerala.

Located near the famed Hindu temple at the Sabarimala hills, the post office may perhaps be the only one in the country which doesn’t work round the year. It comes alive when the peak pilgrimage season of the Ayyappa shrine begins on the first day of the Malayalam month in November, and the period ends towards the middle of January.

It is a unique post office, and one of its main tasks is to deliver letters to God.

The post office is also open for 10 days during the Vishu season.

source: http://indiatoday.intoday.in/story/post-office-kerala-which-delivers-mail-letters-to-god-ayyapa-sabarimala-vishu/1/406330.html