If your schedule varies from week to week, use Specific Dates mode. This lets you set availability for individual dates rather than a recurring weekly pattern.
Freelancers with varying schedules
Part-time consultants
Seasonal availability
Event-based scheduling
Go to Availability from the sidebar
Select Specific Dates mode
Click on dates in the calendar to add availability
Set the hours for each date
Click Save
Add - Click any date to add availability
Edit - Click an existing date to modify hours
Remove - Click the X to remove availability
Remember to add availability for future dates regularly. If no dates are set, clients won't see any available times.