Arriving at the end of each year (that is, the last day of the lunar calendar) wasn't just a new day taking over from the previous one, but also a new year taking over from the previous one, and this was called 'first exchange'.
到了每年的年底即年三十的时候，就不仅是新旧两天的更替，也是新旧岁的更替，称之为“交子”。