This can have various reasons, which is why your RTP calendar now supports not only "Sold out" but also "Closed" text labels.

When will the closed label appear, and when will the sold-out label appear?

Closed: This label is connected to your opening hours and closing times. If your opening times are Tuesday-Sunday, then your closed label will show on Monday.

Sold out/not available: This label is, just like before, connected with your inventory and will show if tickets were available but are not anymore: hence they are sold out. 

How to edit your closed label:

Just like any other copy in your Checkout, you can edit your closed label using your Copy editor.
For the Closed label, you need to search for the following key: rtp_calendar.rtp_closed_label 
For the Sold out label, you need to search for the following key: rtp_calendar.not_available

Do you want to recap how to use the Copy editor? Head over here.

Pro tip: Hack 3 of our favorite copy editor hacks will also apply here - Use emojis to make your Checkout clear at first sight.