A condition for these activities and with precise vacation planning also no waste of vacation days. Vacation planning is done from October to December for the following year.
Even if you don't have to staff an entire production line...sometimes it's simply better to shut down completely than to appoint "volunteers" in every team to cover the rest, where the coverage is then not really needed anyway because almost no one is at the customer during that time. We are also completely closed between Christmas and New Year's.
And in production companies, such a shutdown period is also gladly used to maintain the equipment. An acquaintance of mine has a company that performs such maintenance, and it is often booked for the Christmas/New Year period.