Today is Mahalaya. If you do a Google Search on "what is Mahalaya?" you'll get results such as,
"Mahalaya marks the beginning of Navratri and Durga Puja."
"It is the end of Pitra Paksha and the beginning of the Devi Paksha."
"Mahalaya is the last day of the Pitra Paksha or the period when Hindus remember and offer homage to their deceased ancestors."
"It is the beginning of Durga Puja festival which is the major festival of Bengal."