To create a reliable and consistent calendar we have to have an accurate answer to the question:
‘how many times does the Earth rotate (a day) during one orbit around the Sun (a year)?’
We now know that the Earth takes just over 365 (365.2422) days to orbit the Sun. But we haven't always known that number accurately, and not knowing it accurately can cause real problems.
To find out about a good calendar click here
To find out about the best calendar so far click here
To find out about a simple, but BAD, calendar click here