In this Tamil calendar year called Hevilambi, the full moon day in the Tamil month of Chithirai occurs on 20th of Chithirai - (10th and 11th May). The Chitra Pournami period starts at 1.46 AM on 10th May and ends at 3.27 AM on 11th May.
The best auspicious time for the devotees to take girivalam (Going around the mountain) is 2.00 AM to 4.00 AM on 10th May.
Another significant point is that 2.00 AM to 4.00 AM being the very early morning period, it is believed that the legendary Siddhas are supposed to move around this area during this period. One more interesting point to note is that 2.00 AM to 4.00 AM being a very early morning, it will protect the devotees from the hot summer sun.