A lot of businesses have traditionally closed for a couple weeks for vacations. It makes a lot of sense:
1. Everyone knows when they can take their vacation. For couples who both work there, this is fantastic.
2. Long time customers know it, and can work around it fairly easily..
3. With the plant shut down, a real upgrade/replacement of equipment can take place, not a rushed, "get it done ASAP" rush job.
I worked at a place that did this and it was great, no "I can't get off that week, nobody is available to take my place!". You knew a year ahead of time so you could make plans, etc. They did have a minimal "doomsday" crew, mostly senior management, there during the shutdown. They could do some of the stuff needed, but they were pretty helpless.