After the sun sets, the period just before the ensuing dawn is called Bramhamuhurtham according to vedic scriptures. It is an auspicious time for it gives good thoughts and intentions. Practices like meditation and penances done during this hour, gives favorable results. According to Vedic calendar, 1 Earth-year is equal 1 celestial day. So the last 6 months on the earth (every year) is equal to 1 celestial night. Hence, the predawn period, the Bramhamuhurtha, for the heavenly beings falls in the month of Dhanurmasam. Andal taught us this Dhanurmasa Vratham because she wanted us to spend these auspicious days in contemplation of God. This vratham was performed for Krishna.
According to the poem God is only one and he appears in various forms for our sake. He accepts our offerings and blessings even when he is worshipped in an idol-form in our house.