The census should have recorded people present at each dwelling at midnight at the end of the stated day. In later years the rules were extended to include those who returned to the dwelling the following morning, having not been counted elsewhere, such as "those travelling or out at work that night". This was included in the enumerators instructions for the 1871 census.
Census records, along with civil registrations, are one of the main sources of information for family historians and often a good place to start your family history research. They also give an insight into the lives of our ancestors as you trace their movements and changes in occupation over time.