Your usual practice hours are set up on the Locations page; however, you can add additional hours to any day from the Calendar screen. These hours apply only for the day and will not affect your regular schedule.
To add custom hours for a specific day:
Click the cogwheel icon next to the relevant day on your calendar:
If you have multiple locations, choose the correct location from the drop-down box, then click Add New.
Add the additional hours and click Save.
RESULT: Your calendar will now show additional availability on the day you specified.
Custom availability can only be used to add additional hours; and you cannot remove hours using this function.
If you have a longer span of time with different practice hours, you way wish to add another schedule in the Locations page instead, with the schedule set to apply only between specific dates.