Sandhi Puja is a very significant ritual during Navratri Puja. It is performed at the point when Ashtami Tithi ends and Navami Tithi begins. This is believed to be the moment when Devi Chamunda appeared to kill the demons Chanda and Munda.
Sandhi Puja is dominant for two Ghatis or approximately lasts for 48 minutes. It is possible that Sandhi Puja Muhurta occurs at any time of the day and Sandhi Puja should be done during that time only.
You might also refer the complete Shardiya Navratri Calendar and Durga Puja Calendar.
Navratri Day 1: Thursday 7th October 2021
Pratipada
Ghatasthapana
Chandra Darshan
Shailputri Puja
Navratri color of the day – Yellow
Navratri Day 2: Friday 8th October 2021
Dwitiya
Brahmacharini Puja
Navratri color of the day – Green
Navratri Day 3: Saturday 9th October 2021
Tritiya
Sindoor Tritiya
Chandraghanta Puja
Kushmanda Puja
Varad Vinayaka Chauth
Navratri color of the day – Grey
Navratri Day 4: Sunday 10th October 2021
Panchami
Upang Lalita Vrat
Skandamata Puja
Navratri color of the day – Orange
Navratri Day 5: Monday 11th October 2021
Shashthi
Saraswati Awahan
Katyayani Puja
Navratri color of the day – White
Navratri Day 6: Tuesday 12th October 2021
Saptami
Saraswati Puja
Kalaratri Puja
Navratri color of the day – Red
Navratri Day 7: Wednesday 13th October 2021
Ashtami
Durga Ashtami
Mahagauri Puja
Sandhi Puja
Navratri color of the day – Royal Blue
Navratri Day 8: Thursday 14th October 2021
Navami
Maha Navami
Ayudha Puja, Navami Homa
Navratri color of the day – Pink
Navratri Day 9: Friday 15th October 2021
Dashami
Navratri Parana
Durga Visarjan, Vijayadashami
Navratri color of the day – Purple