Sandhi means junction and in this case it is the junction between the eighth and ninth lunar day after the first new moon that occurs after the sun moves into Virgo -- as per the Hindu zodiac, which lags the Christian zodiac by the Ayanamsa value of around 23 degrees. That is why the ritual is performed in the last 24 mins of the Ashtami ( the 8th lunar day ) and the first 24 mins of Navami (the 9th lunar day).
Legend says the Goddess in her manifestation of the Mahishasur-mardini, the destroyer of the evil ogre who took the form of a buffalo or Mahish, performed her deed at this precise moment and so at this auspicious hour the manifestation of divinity is at its peak in the clay image that is generally used to represent the Goddess.
The actual killing of the ogre -- the triumph of the righteous and the destruction of all that is evil -- is supposed to be remembered and recalled through an actual blood sacrifice but today, most puja organisers settle for symbolic sacrifice which is the climax of this specific ritual.
The precise moment when the Goddess kills the ogre is marked by the setting off of firecrackers -- in faint resemblance of cannons being fired in the past -- and this creates a very unique audio ambience especially in the rural, and hence quieter ! parts of the country. As the priest in each puja performs the symbolic sacrifice and the drums and firecrackers that accompany him burst out as loudly as possible, it seems as if a wave of sound is travelling across the (usually) darkened landscape. The sound moves from puja to puja until the entire countryside is agog in a joyous celebration of good over evil. Those who celebrate the puja in the noisy environs of a city very often miss out this experience.
This short movie on the Sandhi Puja at Sonartoree, Prantik Birbhum shows the event and the spray of blood -- in the form of coloured seeds!