You can actually open your hours for any individual day via your App, even if you don't usually work on that day.
Press the menu in the top left and go onto Profile.
Once there, click Time Off, then click the blue Book time off button at the bottom of the screen (we know that you are not actually booking time off, but you still need to click that button to change your hours)
Select Part-Day and then just enter what time you can start that day and what time you need to be finished by on that day
You can extend your availability on any day that you like and the change will only be made for that one day (not every week).
The only thing you can't use this feature for is booking off the middle of a day and having jobs either side - if you want to do that then you just need to contact Housekeep.