Nishekam is the ritual associated with first sexual intercourse among Hindu married couples.
In popular practice, the bride and the groom have a special arrangement where they engage in sexual activity on the very first night at a pre-determined auspicious time.
[1] Majority of the North Indians follow Shukla Yajurveda, so they do it on 4th day after Vivaham (marriage).
[2] Whereas other kalpasutras mention it just after Griha Pravesham, which occurs next day of marriage in many areas.
It is similar to the preparatory ritual for Garbhadhana ceremony in many aspects as it allows the couple to prepare for begetting a child.