The Puja timings, being based on the lunar month, often spread over to the next day or night. Sashthi is the day for the inauguration, though the main part of the Puja spreads over four days from the day of Saptami to the day of the Bijoya Dashami on which the immersion takes place. So it may not match, as is the case this year, with the days of the Calendar we follow.