One would imagine, as with all things PC, that unilateral decisions are inappropriate.
Where a published schedule of PC meetings exists - and where it is constructed from the instructions laid out in (model) Standing Orders - a deviation from published schedule would only be credible if supported by an (open to the public) meeting where a motion and a vote of the PC were conducted.
Notwithstanding sudden, unforeseen major catastrophe impacting upon the entire community, is a last minute (and I mean literally minutes before the published scheduled meeting is due to commence) unannounced change of PC meeting date lawful / appropriate / acceptable?
It must be assumed that all members of the PC and the clerk were aware in advance, but that it was not communicated to the electorate.