Opening times are a critical part of setting up your venue, but they work a little different to the opening times that you tell customers. You can access this through your the main nav bar settings page.
To make anything bookable on your form you need both a session created and opening times applied.
Basic Opening Times
You can use the add and copy from buttons to add opening times. Note that you'll need at least one session set up on the sessions page for you to apply them too.

If you have days where the opening time is the same, you can save sometime by setting up just a few days and then using the "Copy From" button to pick they day you want to copy from.
We ask for three times for each session.
Open: When you want to start taking bookings for this session
Last Bookings: The time you want to take the last booking. This is inclusive, so if you pick 3pm, then 3pm will be the last available time, not 2:59PM.
Close: When this session is fully closed.
For example, if you have a simple venue with a single main dining session, you may set the opening and close times as the same your actual venue, and set the last booking an hour before close to avoid people turning up too late.
Advanced Opening Times
Sessions always use a regular time slot interval. Usually this is set to 30 or 60 minutes. You can't set irregular intervals on the sessions page, but you can use opening times to configure exactly when you are bookable without making mulitple sessions.
For example, if you want to have a break between lunch and dinner service, you can use a single dining session but set the opening times up like this. Customers won't see a difference in the bookings but won't be able to book between, because we are closed.

You can also use to open and close separate areas of your venue by giving each area it's own session. You can have any amount of opening times.
You can also use opening times for setting up lunch or dinner sessions, or anything else where you want to move times around. A single session can have multiple opening times, and there's no limit to the amount of opening times you have per day:
Events and Opening Times
If you can't see a session on the opening times dropdown menu, it' ls because that session is setup as an event, not a repeatable session.
An event will either show alongside your specified opening hours, or, if set to exclusive, will show instead of your opening hours.
For events, you change the time on the session settings itself. You should always use events for one off, unique or very uncommon sessions, and reputables for everything else.