OP, the law in this case:
Traffic signs
18.—(1) Where an order relating to any road has been made, the order making authority shall take such steps as are necessary to secure—
.....that adequate information as to the effect of the order is made available to persons using the road;
With parking places the norm is that they are self-contained and that each sign conveys all the required info.
But this is not required by law i.e. the boundaries of adequate have been widened to include information which affects parking places being permitted to be displayed on CPZ entry signs where these exist.
You were parked within a CPZ whose signs would ordinarily have no effect on parking places within. Except that if 'event day' info was displayed prominently on each then this has been held to meet the test of 'adequacy'.
You do not need to know what is an 'event', this isn't your burden. The council must display adequate information which informs a motorist of 'event days' however 'event' is defined.
Hence why you are being asked about your route so that the signs you passed can be identified.