Compulsory shutdown is when you are forced to take some annual leave at a particular time. This often occurs in the manufacturing industry when they have a furnace that they turn off, wait days for it to cool down, perform maintenance then it takes a couple more days to heat up again. During this time no-one in a chain of work involving the furnace can do anything. As this often was the majority of staff this has carried through to the office staff of the 20th and 21st centuries.
On another note, I tend to find that working freelance on a daily rate has meant that clients now rather time wasn't taken when deadlines are tight. Before, on an hourly rate, I could check that they were happy for me to work a little longer each day to ensure that the necessary tasks were achieved on time then take a day off. To be honest, I am not prepared to do work extra unpaid hours to fit deadlines that ignore room for absences (i.e. holiday/vacation). I am prepared to put in extra hours when required but not to take a days leave unpaid. PLEASE NOTE: that it is not all companies that are so inflexible.
-- Stop your grinnin' and drop your linen...they're everywhere!!!