Chinese New Year is celebrated on the first day of the first moon. Just as Christmas was not always celebrated on 25 December, in ancient times the date of the Chinese New Year varied, for every emperor regarded it as his royal prerogative to change the calendar upon accession to the throne. By royal command, astronomers were invited to make a new calculation which, nevertheless, had to correlate with the sowing of seeds and subsequent planting of crops by the farmers. The royal proclamation that followed was a major event, for on this new calendar depended the success of agriculture, and hence the well-being of the people which, in turn, would ensure that the emperor continued to have the mandate of heaven to rule the country. Ever since the reform of the lunar calendar in 104 BC, however, the Chinese New Year always occurs between 21 January and 19 February.