You don't need to set a day...if none are selected, it will happen the next time that TOD occurs, one time only...but, the actual time will stay in the field. If you want to use that time again, then, you can just click on the box to activate it. Not having any particular schedule, I do not have any days selected and I find it works fine. If you want to set up for something by skipping a day or so, then you'd need to select a day for it to occur.
For optimal operation, the departure time should be at least 3, and preferably 4 hours in the future if it's really cold out. You can enter it anytime, but you may not benefit fully, since the battery may not have time to be fully conditioned. On a moderate day, it may not start doing much until nearly your departure time, if the battery is near ideal temp already.
For maximum efficiency, you might also set in a time window, even if you don't have off-peak charging , so that the window would mean the car would be charging before you left. When it's cold out, the act of charging will warm them, and the car may not need to use it's heater as much. YOu may need to play a bit with that so that there's enough time to actually get a full charge. I do not have off-peak charging where I live, so I've not played with that feature.
2011 535i x-drive GT, 2014 i3 BEV
Soon (hopefully!) A 2021 X5 45e will replace the above