Without the comma, "which" is defining, i.e. it tells us which thing is meant. With the comma, "which" is non-defining, i.e. it gives more information about something that has been mentioned. However, there is another issue here, namely whether "which" refers to a noun or to a whole statement. In your first sentence, "which" refers to the noun "date" or "event". In the second sentence, "which" ref