Occasion:
Nichayathartham or Engagement marks the coming together of the bride and the groom’s family , led by their respective elders where they commit to each other about the wedlock they are about to enter .
Purpose & Benefits:
It forms a very special occasion in the ceremonies conducted in a traditional marriage because it graces the occasion by endorsing their commitment towards starting a new family . It may be done either in the groom’s place or in the Bride’s place and any decision is taken only after the important members are convinced about the proceedings. It is an indispensable practice which adds impetus to any marriage .The customs are well scrutinized and is aimed at getting the choicest blessings from the gods and deriving the well wishes of all near and dear ones.
Timing:
It is performed before the marriage in any auspicious month that conforms with the auspicious alignments of the couple’s horoscope.
Ceremony Description:
The engagement ceremony for the Tamil Brahmins (Iyers and Iyengars) basicaly follow the same rituals and customs but there are slight variations. For instance, the engagement ceremony in the case of the Iyers takes place in the groom’s residence whereas the Iyengars follow the custom of hosting it in the bride’s home. A ‘Vadhyar’ (Brahmin priest) writes the ‘lagna patrika’ specifiying the date, time and venue of the marriage and this serves as a sort of commitment for both families. The bride’s family brings fruits and sweetmeats and the groom’s people in turn serve their family members snacks and dinner and present a sari to the bride.
Source harivara.com