You set up your availability for a regular week in the Availabilities. However, in real life, last-minute things come up. Let's say you need to attend a function this coming Thursday. neetoCal provides Availability overrides to take care of these situations.
If your schedule changes a lot from week to week, check out Handling different schedules each week.
Click on Availabilities from the left-hand sidebar.
Click on the availability you want to add an override.
Click on Add next to Availability overrides.
Add the hours you are available.
Click on Submit.
Once the availability override is set, then for those specific days, the override takes precedence over the weekly availability.