Every God and Goddess in Hindu religion has a particular day associated with them. The day associated with Ayyappa or Sabarimala Sastha is Saturday. It is widely believed that Ayyappan appeared on earth to satisfy his devotees (born) on the last day of Dhanu Masam as per traditional Malayalam Hindu solar calendar followed in Kerala. The day was Saturday. The nakshtra was Uthram and the tithi was Krishna Paksha Panchami.
It is also believed that the murti of Ayyappa was consecrated at Sabarimala Ayyappa temple on a Saturday on Makara Samkramana day. The tithi was Krishna Paksha panchami and nakshatra was uthram.
It is widely believed that having vegetarian food only once on the day and visiting Ayyappa temple will help a devotee in achieving desire fulfillment.