Something that might work ...
- You could edit the configuration of one of the extensions that is in the Ring group via the 'Office Hours' tab
- change its own Office Hours settings to reflect your weekend schedule
- select the tick-box to Automatically switch office hours profiles,
- and have that extension configured to go directly to the required voice-mail box when it's in 'Out of Office' mode.
With this approach, its possible to build a third-state rule that allows that extension to 'hijack' the ring group during those hours ... In other words, because one of its member extensions has changed state to answering & sending calls directly to voice-mail, the ring group no longer gets a chance to handle the call!
I hope that's not too confusing? ... best regards ...