The main legend is from Mahabharata.
When the Pandavas were in exile, Lord Krishna gave Draupadi a bowl called the Akshaya Pathram. The word Akshaya means that which never diminishes and this bowl is said to have given infinite food to the Pandavas.
This is why this day is considered to be a good time to bring home Goddess Lakshmi, in any form, gold, silver or platinum.
Valuables that are acquired on this day are said to be replenished endlessly.