The cause of this may be because this is the one time of year people in Germany can actually get their hands on fireworks. Whereas small fireworks can be sold to people in Germany all year round, stores are only allowed to sell larger fireworks - the kind you're likely to set off on Silvester - between December 28th and December 30th. What's more – you're only really allowed to set off fireworks yourself between the 31st of December and the morning of the 1st of January. According to German law, Klasse 2 fireworks should not be launched in the immediate vicinity of hospitals, churches or old people's homes. Apart from around these locations, you're free to set off as many Klasse 2 fireworks as you'd like on December 31st.

Source: The Local December 27, 2019 10:18 UTC