Mitra saptami is observed on Shukla Paksha Saptami during Margasira masam or Margashirsh month. Mitra Saptami date 2015 is December 18.
This festival is dedicated to Surya Bhagawan or Sun God. Surya Bhagwan is worshipped with different name during each month. In Margashira masam, Sun God is worshipped as Mitra, hence the Saptami day on which Surya is worshipped as Mitra Saptami.
On Mitra Saptami, Surya bhagwan is offered prayers and special naivedyam. Devotees believe that worshipping Lord Surya on this day would provide them with good health and wealth. On this day, devotees avoid using hair oil.