On Sunday, 2/15 at 20:00 GMT (noon PST), we are going to be making some code changes that will log you out of your WordPress.com. If you have unsaved work at the time the change is made, it is possible you will lose it since you will have to log in again before you will be able to access your dashboard. I recommend saving your work shortly before the maintenance begins just to be safe.
There is nothing you need to do to prepare for this. You do not need to backup your blog. All of your data is safe.