Durga puja is a mega event, it is celebrated across several days. Most importantaly it starts from Mahalaya and ends with Dashami (bisarjan)
Goddess Durga is invited to come to Earth on this day. And this is the begining of the mega occasion. Mahalaya has other signicance also.
This day the marks the begining of the Durgapuja, all the pendals are ready by this day.
Shashti is when Goddess Durga descends to earth with her four children Lakshmi, Saraswati, Kartikeya and Ganesh.
Saptami is the first day of the durga puja.
Ashtami Puja day is the most significant day of the celebration.
Maha Navami or Navami is 2nd last day of the celebration. Goddess Durga is believed to have killed demon Mahishasura on this day.
Dashami or Vijaya Dashami is the last day of the durga puja. This day Goddess Durga returns to her husband's abode and the idols are taken for immersion.