It's not an explicit option, but there is a way you can do it.

Instead of routing to a queue, send it to an extension instead (like a receptionist extension). Add your message in the "User Greeting" and then add your after hours service number in the call handling "missed calls" section.

Thanks for you response @tak-it1 -- when you say 'user greeting', do you mean the voicemail greeting? I'm not sure I can easily locate where to make these setting changes.

It seems like the extension settings would need to be updated in real time, and cannot be set for a scheduled amount of time like the custom rules. Is that right? For example, we'd have to go in and add the user greeting from the receptionist extension at the beginning of the holiday, and remove it at the end of the holiday manually, to avoid it playing for calls outside of the holiday timeframe?

Thanks again, appreciate your help.

" I'm not sure I can easily locate where to make these setting changes."
Been using RC as an admin for 3+ years and have the same issue since the UI re-design.

- It seems like the extension settings would need to be updated in real time, and cannot be set for a scheduled amount of time like the custom rules.

It's possible. To sum up the entire set up:
- you need 2 custom rules.
- custom rule1 redirects calls on your main number to the receptionist extension during the holidays. This probably is created under Phone System > Auto receptionist, but it actually depends on where your main number is assigned to.

- custom rule2 changes the call handling for the receptionist extension during the holidays. Go to Users > click on the receptionist ext. > Phone > Call rules and voicemail > +Add rule > Create custom Rule

use these parameters for custom rule2
Name: whatever you want
Conditions: add schedule > Specific date range > holidays
Call handling: select Ring my devices and configure appropriately

and that's it !