Though not essential for survival, ritual is important for tying together communities. The participants create the meaning of a ritual: they understand its importance and enjoy helping create a special sense of occasion. Examples that spring to mind are giving presents, dancing, celebrating special days and celebrating rites of passage. But there are hundreds of day-to-day behaviours or conventions that you could probably classify as ritual.
Some angles I'll cover are:
Ritual per se
Solstice
Theatre