Off Topic is a special treat. It usually occurs near or on a holiday and it can be open for anywhere from a day to a couple of weeks. But, it can be any time so chosen. It is our moderators who have the call on when to open, and when to close.
I enjoy it immensly. Sometimes, I wish it were here all the time, but, I guess that, then, it wouldn't be so special, and fun.
My guess is that the extended length for this one is because of the holiday period (Xmas going into New Year). Now that we've made it past NY Day, it'll probably close very soon.
They usually open on the holiday (whichever holiday it is) and close within a day or so of the holiday/holiday weekend ending. Sometimes the mods will open without a holiday, as they did during the recent storms in the east.