The Arattu is observed on the 4th pada or the last quarter of Amavasya day. So the festival might extend to the 9th day.
Maha Sivaratri, celebrated all over India , falls on the 13th (or 14th) day of the dark half of the Phalgun (February-March) month. The name "Maha Sivaratri" means "the night of Lord Siva" and celebrations happen all over the country on the night of this auspicious occasion.
Once the Devas and Asuras joined together for "Saagaramathanam" - churning of Ocean. They enjoyed PariJatha, KalpaVrikshu (Tree which bestows any boons), Kamadhenu (Cow which bestows any boons) and Amritha (Nectar). However when the mighty poison "Kalakoota" emerged, all of them were in panic and nobody could combat it.
At that time Lord Shiva emerged to help the world and swallowed the poison. Goddess Parvati was all for her husband, held his throat . Lord Siva neither swallowed nor spat out the poison as he held it in his neck and became Neelakanta (Blue neck)
Maha Sivaratri is supposed to be the favorite day of Lord Shiva. Devotees take Sivaratri vrata to control the negative emotions of passion and ignorance in them. All devotees fast the whole day, many of them without taking even taking a single drop of water.
They will perform a grand havan for the peace and welfare of all. The whole day will be spent in chanting "Om Namah Shivaya" and in meditation upon the Lord. Shiva temples all over the country will conduct special poojas and during the four quarters of Sivaratri, the Shiva Lingam is worshipped with intense devotion. Sannyas Diksha is given on this day to earnest seekers on the path.