We are staying here until Friday.
there is no need for a 'breather' comma to help the reader. Is there a syntactic need or a sensible reason for inserting a comma there?
There possibly is – it depends on whether you want to emphasise the temporal adverbial or not.
We are staying here until Friday. is the unmarked, 'monotone' version – a simple statement of fact.
We are staying here, until Friday.
We are staying here – until Friday.
We are staying here. Until Friday.
progressively set off and increasingly emphasise the temporal adverbial.
Setting off a temporal modifier starting with 'until' after a negative statement gives an improbable dramatic change of direction to the sentence – a device that would only be used for such a dramatic effect.
I will not be firing any employees. Until Friday.