The most fundamental function of OwnerRez is to make sure all your calendars are properly synchronized. The whole point of OwnerRez' calendar features is to make sure that, when a booking comes in, it doesn't matter how the booking is placed - all your calendars, everywhere, are correctly blocked off so you don't ever get double bookings.
But in order for this to reliably work, it's essential to test all the various possible combinations to make sure you haven't missed anything.
Hosted Website and Widget Bookings
First, some good news: it is not necessary to specifically test the calendar on your Hosted Website or displayed in OwnerRez Widgets on your own website. Nor do you need to verify that bookings made via those methods. Both Hosted Websites and Widgets run directly off of the master OwnerRez database, and will always display whatever is visible on your main Booking calendar display in OwnerRez. It is not possible for them to display anything else - there's no caching or duplicated records anywhere that can get out of sync, just one master database controlling all of the above.
So whenever you are testing calendars, you can consider the main OwnerRez Booking display to fully and completely represent your Hosted Website and/or all Widgets at all times.
In the unlikely event that you are using OwnerRez entirely on its own, connected to no channels other than a Hosted Website and/or Widget-based website, congratulations! You don't need to do any calendar testing.
Just about all OwnerRez clients have their properties listed on multiple channels, and testing to make sure each calendar is syncing correctly is essential.
For purposes of calendar testing, it doesn't matter whether you are connected to your channels using an API connection or iCal calendar syncs - testing is the same either way. (Fixing any problems you may find will be different though.)
💡 Special Note about Vrbo API
If you are using the Vrbo API connection, the API handles information for all bookings that are placed after you turn on the API. It ignores any bookings that previously existed.
For this reason, even after setting up the Vrbo API, you still need to have your iCal calendar syncs set up between OwnerRez and Vrbo in both directions for all properties. The iCal connections are needed in case the dates of any old bookings were to change. All new bookings, and any changes to bookings created after the API is turned on, will be handled by the API.
The only way to test the iCal connections would be to actually change the dates of an old booking, and you really don't want to do that. So if you are using Vrbo and are about to set up an API connection, it's important to first set up your iCal calendar syncs and test them, because you won't have a way to do that later.
To fully test your calendar connections, you need to test both calendar import and calendar export, for each property, and for each channel. That's a lot of testing! It might be useful to make a list of all the possible combinations and work your way down, checking things off one at a time.
If you are using Mutual Blocking, don't forget to check the properties that should automatically be blocked when other properties are booked!