Devipaksha is the waxing phase of moon in Ashwin month (September – October) as per traditional Hindu lunar calendar. In 2012, Devipaksha is from October 15 to October 24 – Navratri and Durga Puja are held during this period. The Devi Paksha begins with the Mahalaya Amavasya – it is said that the gods and goddess awake on this day to prepare for durga pooja.
It is said that Goddess Durga starts her journey towards the earth of the first day of the Devipaksha period.
It is believed that Goddess Durga was originally worshipped (Durga Puja) in the Chaitra month (March – April) and was also referred as Basanti Puja. It was Lord Ram who changed the period of Durga Puja.
Lord Ram wanted to get the blessings of Goddess Durga before beginning the war with Ravana. Therefore he invoked Goddess Durga during the Devipaksha in Ashwin month (October – November). This is why the Durga Puja during Ashvina month is also known as Akal Bodhon or untimely invocation.