The nikah (a religious marriage contract) takes place either on Shaadi day or any other day.  It is the most essential part of a Muslim marriage as without nikah it will not be considered as marriage. All other events of the marriage are not essential and there is no value or importance for those events in Islamic point of view and a marriage can be take place without them but without nikah it will not be called marriage. In most cases the venue for nikah is bride’s house however some people do it in a mosque. It is performed by an imam.  During nikah both bride and groom must have their witnesses present just to ensure that the marriage is consensual. In fact nikah can not be performed without the presence of witnesses. Also both bride and groom should be agreeing for it otherwise it will not be called a valid nikah. After nikah groom and bride can be called husband and wife.