The day on which you give in to your computer's non-stop reminders and install lots of updates at once. No work may be done on the computer on this day, hence it being a "Sabbath."
Boss: Can you type up a letter to the client before the end of the day?
Employee: No can do, chief. I still have 23 critical updates for Windows to install.
Boss: Ah, it's Download Sabbath. I see.